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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>3. Data model — Python 2.7.16 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.16 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="4. Execution model" href="executionmodel.html" /> <link rel="prev" title="2. Lexical analysis" href="lexical_analysis.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/2/reference/datamodel.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="executionmodel.html" title="4. Execution model" accesskey="N">next</a> |</li> <li class="right" > <a href="lexical_analysis.html" title="2. Lexical analysis" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.16 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">The Python Language Reference</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="data-model"> <span id="datamodel"></span><h1>3. Data model<a class="headerlink" href="#data-model" title="Permalink to this headline">¶</a></h1> <div class="section" id="objects-values-and-types"> <span id="objects"></span><h2>3.1. Objects, values and types<a class="headerlink" href="#objects-values-and-types" title="Permalink to this headline">¶</a></h2> <p id="index-0"><em class="dfn">Objects</em> are Python’s abstraction for data. All data in a Python program is represented by objects or by relations between objects. (In a sense, and in conformance to Von Neumann’s model of a “stored program computer,” code is also represented by objects.)</p> <p id="index-1">Every object has an identity, a type and a value. An object’s <em>identity</em> never changes once it has been created; you may think of it as the object’s address in memory. The ‘<a class="reference internal" href="expressions.html#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a>’ operator compares the identity of two objects; the <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-func docutils literal notranslate"><span class="pre">id()</span></code></a> function returns an integer representing its identity (currently implemented as its address). An object’s <em class="dfn">type</em> is also unchangeable. <a class="footnote-reference" href="#id5" id="id1">[1]</a> An object’s type determines the operations that the object supports (e.g., “does it have a length?”) and also defines the possible values for objects of that type. The <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a> function returns an object’s type (which is an object itself). The <em>value</em> of some objects can change. Objects whose value can change are said to be <em>mutable</em>; objects whose value is unchangeable once they are created are called <em>immutable</em>. (The value of an immutable container object that contains a reference to a mutable object can change when the latter’s value is changed; however the container is still considered immutable, because the collection of objects it contains cannot be changed. So, immutability is not strictly the same as having an unchangeable value, it is more subtle.) An object’s mutability is determined by its type; for instance, numbers, strings and tuples are immutable, while dictionaries and lists are mutable.</p> <p id="index-2">Objects are never explicitly destroyed; however, when they become unreachable they may be garbage-collected. An implementation is allowed to postpone garbage collection or omit it altogether — it is a matter of implementation quality how garbage collection is implemented, as long as no objects are collected that are still reachable.</p> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> CPython currently uses a reference-counting scheme with (optional) delayed detection of cyclically linked garbage, which collects most objects as soon as they become unreachable, but is not guaranteed to collect garbage containing circular references. See the documentation of the <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module for information on controlling the collection of cyclic garbage. Other implementations act differently and CPython may change. Do not depend on immediate finalization of objects when they become unreachable (ex: always close files).</p> </div> <p>Note that the use of the implementation’s tracing or debugging facilities may keep objects alive that would normally be collectable. Also note that catching an exception with a ‘<a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>…<a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a>’ statement may keep objects alive.</p> <p>Some objects contain references to “external” resources such as open files or windows. It is understood that these resources are freed when the object is garbage-collected, but since garbage collection is not guaranteed to happen, such objects also provide an explicit way to release the external resource, usually a <code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code> method. Programs are strongly recommended to explicitly close such objects. The ‘<a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>…<a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a>’ statement provides a convenient way to do this.</p> <p id="index-3">Some objects contain references to other objects; these are called <em>containers</em>. Examples of containers are tuples, lists and dictionaries. The references are part of a container’s value. In most cases, when we talk about the value of a container, we imply the values, not the identities of the contained objects; however, when we talk about the mutability of a container, only the identities of the immediately contained objects are implied. So, if an immutable container (like a tuple) contains a reference to a mutable object, its value changes if that mutable object is changed.</p> <p>Types affect almost all aspects of object behavior. Even the importance of object identity is affected in some sense: for immutable types, operations that compute new values may actually return a reference to any existing object with the same type and value, while for mutable objects this is not allowed. E.g., after <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">=</span> <span class="pre">1;</span> <span class="pre">b</span> <span class="pre">=</span> <span class="pre">1</span></code>, <code class="docutils literal notranslate"><span class="pre">a</span></code> and <code class="docutils literal notranslate"><span class="pre">b</span></code> may or may not refer to the same object with the value one, depending on the implementation, but after <code class="docutils literal notranslate"><span class="pre">c</span> <span class="pre">=</span> <span class="pre">[];</span> <span class="pre">d</span> <span class="pre">=</span> <span class="pre">[]</span></code>, <code class="docutils literal notranslate"><span class="pre">c</span></code> and <code class="docutils literal notranslate"><span class="pre">d</span></code> are guaranteed to refer to two different, unique, newly created empty lists. (Note that <code class="docutils literal notranslate"><span class="pre">c</span> <span class="pre">=</span> <span class="pre">d</span> <span class="pre">=</span> <span class="pre">[]</span></code> assigns the same object to both <code class="docutils literal notranslate"><span class="pre">c</span></code> and <code class="docutils literal notranslate"><span class="pre">d</span></code>.)</p> </div> <div class="section" id="the-standard-type-hierarchy"> <span id="types"></span><h2>3.2. The standard type hierarchy<a class="headerlink" href="#the-standard-type-hierarchy" title="Permalink to this headline">¶</a></h2> <p id="index-4">Below is a list of the types that are built into Python. Extension modules (written in C, Java, or other languages, depending on the implementation) can define additional types. Future versions of Python may add types to the type hierarchy (e.g., rational numbers, efficiently stored arrays of integers, etc.).</p> <p id="index-5">Some of the type descriptions below contain a paragraph listing ‘special attributes.’ These are attributes that provide access to the implementation and are not intended for general use. Their definition may change in the future.</p> <dl class="docutils"> <dt>None</dt> <dd><p class="first last" id="index-6">This type has a single value. There is a single object with this value. This object is accessed through the built-in name <code class="docutils literal notranslate"><span class="pre">None</span></code>. It is used to signify the absence of a value in many situations, e.g., it is returned from functions that don’t explicitly return anything. Its truth value is false.</p> </dd> <dt>NotImplemented</dt> <dd><p class="first last" id="index-7">This type has a single value. There is a single object with this value. This object is accessed through the built-in name <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>. Numeric methods and rich comparison methods may return this value if they do not implement the operation for the operands provided. (The interpreter will then try the reflected operation, or some other fallback, depending on the operator.) Its truth value is true.</p> </dd> <dt>Ellipsis</dt> <dd><p class="first last" id="index-8">This type has a single value. There is a single object with this value. This object is accessed through the built-in name <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code>. It is used to indicate the presence of the <code class="docutils literal notranslate"><span class="pre">...</span></code> syntax in a slice. Its truth value is true.</p> </dd> <dt><a class="reference internal" href="../library/numbers.html#numbers.Number" title="numbers.Number"><code class="xref py py-class docutils literal notranslate"><span class="pre">numbers.Number</span></code></a></dt> <dd><p class="first" id="index-9">These are created by numeric literals and returned as results by arithmetic operators and arithmetic built-in functions. Numeric objects are immutable; once created their value never changes. Python numbers are of course strongly related to mathematical numbers, but subject to the limitations of numerical representation in computers.</p> <p>Python distinguishes between integers, floating point numbers, and complex numbers:</p> <dl class="last docutils"> <dt><a class="reference internal" href="../library/numbers.html#numbers.Integral" title="numbers.Integral"><code class="xref py py-class docutils literal notranslate"><span class="pre">numbers.Integral</span></code></a></dt> <dd><p class="first" id="index-10">These represent elements from the mathematical set of integers (positive and negative).</p> <p>There are three types of integers:</p> <dl class="docutils"> <dt>Plain integers</dt> <dd><p class="first last" id="index-11">These represent numbers in the range -2147483648 through 2147483647. (The range may be larger on machines with a larger natural word size, but not smaller.) When the result of an operation would fall outside this range, the result is normally returned as a long integer (in some cases, the exception <a class="reference internal" href="../library/exceptions.html#exceptions.OverflowError" title="exceptions.OverflowError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OverflowError</span></code></a> is raised instead). For the purpose of shift and mask operations, integers are assumed to have a binary, 2’s complement notation using 32 or more bits, and hiding no bits from the user (i.e., all 4294967296 different bit patterns correspond to different values).</p> </dd> <dt>Long integers</dt> <dd><p class="first last" id="index-12">These represent numbers in an unlimited range, subject to available (virtual) memory only. For the purpose of shift and mask operations, a binary representation is assumed, and negative numbers are represented in a variant of 2’s complement which gives the illusion of an infinite string of sign bits extending to the left.</p> </dd> <dt>Booleans</dt> <dd><p class="first last" id="index-13">These represent the truth values False and True. The two objects representing the values <code class="docutils literal notranslate"><span class="pre">False</span></code> and <code class="docutils literal notranslate"><span class="pre">True</span></code> are the only Boolean objects. The Boolean type is a subtype of plain integers, and Boolean values behave like the values 0 and 1, respectively, in almost all contexts, the exception being that when converted to a string, the strings <code class="docutils literal notranslate"><span class="pre">"False"</span></code> or <code class="docutils literal notranslate"><span class="pre">"True"</span></code> are returned, respectively.</p> </dd> </dl> <p class="last" id="index-14">The rules for integer representation are intended to give the most meaningful interpretation of shift and mask operations involving negative integers and the least surprises when switching between the plain and long integer domains. Any operation, if it yields a result in the plain integer domain, will yield the same result in the long integer domain or when using mixed operands. The switch between domains is transparent to the programmer.</p> </dd> <dt><a class="reference internal" href="../library/numbers.html#numbers.Real" title="numbers.Real"><code class="xref py py-class docutils literal notranslate"><span class="pre">numbers.Real</span></code></a> (<a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a>)</dt> <dd><p class="first last" id="index-15">These represent machine-level double precision floating point numbers. You are at the mercy of the underlying machine architecture (and C or Java implementation) for the accepted range and handling of overflow. Python does not support single-precision floating point numbers; the savings in processor and memory usage that are usually the reason for using these are dwarfed by the overhead of using objects in Python, so there is no reason to complicate the language with two kinds of floating point numbers.</p> </dd> <dt><a class="reference internal" href="../library/numbers.html#numbers.Complex" title="numbers.Complex"><code class="xref py py-class docutils literal notranslate"><span class="pre">numbers.Complex</span></code></a></dt> <dd><p class="first last" id="index-16">These represent complex numbers as a pair of machine-level double precision floating point numbers. The same caveats apply as for floating point numbers. The real and imaginary parts of a complex number <code class="docutils literal notranslate"><span class="pre">z</span></code> can be retrieved through the read-only attributes <code class="docutils literal notranslate"><span class="pre">z.real</span></code> and <code class="docutils literal notranslate"><span class="pre">z.imag</span></code>.</p> </dd> </dl> </dd> <dt>Sequences</dt> <dd><p class="first" id="index-17">These represent finite ordered sets indexed by non-negative numbers. The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> returns the number of items of a sequence. When the length of a sequence is <em>n</em>, the index set contains the numbers 0, 1, …, <em>n</em>-1. Item <em>i</em> of sequence <em>a</em> is selected by <code class="docutils literal notranslate"><span class="pre">a[i]</span></code>.</p> <p id="index-18">Sequences also support slicing: <code class="docutils literal notranslate"><span class="pre">a[i:j]</span></code> selects all items with index <em>k</em> such that <em>i</em> <code class="docutils literal notranslate"><span class="pre"><=</span></code> <em>k</em> <code class="docutils literal notranslate"><span class="pre"><</span></code> <em>j</em>. When used as an expression, a slice is a sequence of the same type. This implies that the index set is renumbered so that it starts at 0.</p> <p id="index-19">Some sequences also support “extended slicing” with a third “step” parameter: <code class="docutils literal notranslate"><span class="pre">a[i:j:k]</span></code> selects all items of <em>a</em> with index <em>x</em> where <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">=</span> <span class="pre">i</span> <span class="pre">+</span> <span class="pre">n*k</span></code>, <em>n</em> <code class="docutils literal notranslate"><span class="pre">>=</span></code> <code class="docutils literal notranslate"><span class="pre">0</span></code> and <em>i</em> <code class="docutils literal notranslate"><span class="pre"><=</span></code> <em>x</em> <code class="docutils literal notranslate"><span class="pre"><</span></code> <em>j</em>.</p> <p>Sequences are distinguished according to their mutability:</p> <dl class="last docutils"> <dt>Immutable sequences</dt> <dd><p class="first" id="index-20">An object of an immutable sequence type cannot change once it is created. (If the object contains references to other objects, these other objects may be mutable and may be changed; however, the collection of objects directly referenced by an immutable object cannot change.)</p> <p>The following types are immutable sequences:</p> <dl class="last docutils"> <dt>Strings</dt> <dd><p class="first" id="index-21">The items of a string are characters. There is no separate character type; a character is represented by a string of one item. Characters represent (at least) 8-bit bytes. The built-in functions <a class="reference internal" href="../library/functions.html#chr" title="chr"><code class="xref py py-func docutils literal notranslate"><span class="pre">chr()</span></code></a> and <a class="reference internal" href="../library/functions.html#ord" title="ord"><code class="xref py py-func docutils literal notranslate"><span class="pre">ord()</span></code></a> convert between characters and nonnegative integers representing the byte values. Bytes with the values 0–127 usually represent the corresponding ASCII values, but the interpretation of values is up to the program. The string data type is also used to represent arrays of bytes, e.g., to hold data read from a file.</p> <p class="last" id="index-22">(On systems whose native character set is not ASCII, strings may use EBCDIC in their internal representation, provided the functions <a class="reference internal" href="../library/functions.html#chr" title="chr"><code class="xref py py-func docutils literal notranslate"><span class="pre">chr()</span></code></a> and <a class="reference internal" href="../library/functions.html#ord" title="ord"><code class="xref py py-func docutils literal notranslate"><span class="pre">ord()</span></code></a> implement a mapping between ASCII and EBCDIC, and string comparison preserves the ASCII order. Or perhaps someone can propose a better rule?)</p> </dd> <dt>Unicode</dt> <dd><p class="first last" id="index-23">The items of a Unicode object are Unicode code units. A Unicode code unit is represented by a Unicode object of one item and can hold either a 16-bit or 32-bit value representing a Unicode ordinal (the maximum value for the ordinal is given in <code class="docutils literal notranslate"><span class="pre">sys.maxunicode</span></code>, and depends on how Python is configured at compile time). Surrogate pairs may be present in the Unicode object, and will be reported as two separate items. The built-in functions <a class="reference internal" href="../library/functions.html#unichr" title="unichr"><code class="xref py py-func docutils literal notranslate"><span class="pre">unichr()</span></code></a> and <a class="reference internal" href="../library/functions.html#ord" title="ord"><code class="xref py py-func docutils literal notranslate"><span class="pre">ord()</span></code></a> convert between code units and nonnegative integers representing the Unicode ordinals as defined in the Unicode Standard 3.0. Conversion from and to other encodings are possible through the Unicode method <code class="xref py py-meth docutils literal notranslate"><span class="pre">encode()</span></code> and the built-in function <a class="reference internal" href="../library/functions.html#unicode" title="unicode"><code class="xref py py-func docutils literal notranslate"><span class="pre">unicode()</span></code></a>.</p> </dd> <dt>Tuples</dt> <dd><p class="first last" id="index-24">The items of a tuple are arbitrary Python objects. Tuples of two or more items are formed by comma-separated lists of expressions. A tuple of one item (a ‘singleton’) can be formed by affixing a comma to an expression (an expression by itself does not create a tuple, since parentheses must be usable for grouping of expressions). An empty tuple can be formed by an empty pair of parentheses.</p> </dd> </dl> </dd> <dt>Mutable sequences</dt> <dd><p class="first" id="index-25">Mutable sequences can be changed after they are created. The subscription and slicing notations can be used as the target of assignment and <a class="reference internal" href="simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> (delete) statements.</p> <p>There are currently two intrinsic mutable sequence types:</p> <dl class="docutils"> <dt>Lists</dt> <dd><p class="first last" id="index-26">The items of a list are arbitrary Python objects. Lists are formed by placing a comma-separated list of expressions in square brackets. (Note that there are no special cases needed to form lists of length 0 or 1.)</p> </dd> <dt>Byte Arrays</dt> <dd><p class="first last" id="index-27">A bytearray object is a mutable array. They are created by the built-in <a class="reference internal" href="../library/functions.html#bytearray" title="bytearray"><code class="xref py py-func docutils literal notranslate"><span class="pre">bytearray()</span></code></a> constructor. Aside from being mutable (and hence unhashable), byte arrays otherwise provide the same interface and functionality as immutable bytes objects.</p> </dd> </dl> <p class="last" id="index-28">The extension module <a class="reference internal" href="../library/array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><code class="xref py py-mod docutils literal notranslate"><span class="pre">array</span></code></a> provides an additional example of a mutable sequence type.</p> </dd> </dl> </dd> <dt>Set types</dt> <dd><p class="first" id="index-29">These represent unordered, finite sets of unique, immutable objects. As such, they cannot be indexed by any subscript. However, they can be iterated over, and the built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> returns the number of items in a set. Common uses for sets are fast membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference.</p> <p>For set elements, the same immutability rules apply as for dictionary keys. Note that numeric types obey the normal rules for numeric comparison: if two numbers compare equal (e.g., <code class="docutils literal notranslate"><span class="pre">1</span></code> and <code class="docutils literal notranslate"><span class="pre">1.0</span></code>), only one of them can be contained in a set.</p> <p>There are currently two intrinsic set types:</p> <dl class="last docutils"> <dt>Sets</dt> <dd><p class="first last" id="index-30">These represent a mutable set. They are created by the built-in <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-func docutils literal notranslate"><span class="pre">set()</span></code></a> constructor and can be modified afterwards by several methods, such as <code class="xref py py-meth docutils literal notranslate"><span class="pre">add()</span></code>.</p> </dd> <dt>Frozen sets</dt> <dd><p class="first last" id="index-31">These represent an immutable set. They are created by the built-in <a class="reference internal" href="../library/stdtypes.html#frozenset" title="frozenset"><code class="xref py py-func docutils literal notranslate"><span class="pre">frozenset()</span></code></a> constructor. As a frozenset is immutable and <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a>, it can be used again as an element of another set, or as a dictionary key.</p> </dd> </dl> </dd> <dt>Mappings</dt> <dd><p class="first" id="index-32">These represent finite sets of objects indexed by arbitrary index sets. The subscript notation <code class="docutils literal notranslate"><span class="pre">a[k]</span></code> selects the item indexed by <code class="docutils literal notranslate"><span class="pre">k</span></code> from the mapping <code class="docutils literal notranslate"><span class="pre">a</span></code>; this can be used in expressions and as the target of assignments or <a class="reference internal" href="simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statements. The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> returns the number of items in a mapping.</p> <p>There is currently a single intrinsic mapping type:</p> <dl class="last docutils"> <dt>Dictionaries</dt> <dd><p class="first" id="index-33">These represent finite sets of objects indexed by nearly arbitrary values. The only types of values not acceptable as keys are values containing lists or dictionaries or other mutable types that are compared by value rather than by object identity, the reason being that the efficient implementation of dictionaries requires a key’s hash value to remain constant. Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (e.g., <code class="docutils literal notranslate"><span class="pre">1</span></code> and <code class="docutils literal notranslate"><span class="pre">1.0</span></code>) then they can be used interchangeably to index the same dictionary entry.</p> <p>Dictionaries are mutable; they can be created by the <code class="docutils literal notranslate"><span class="pre">{...}</span></code> notation (see section <a class="reference internal" href="expressions.html#dict"><span class="std std-ref">Dictionary displays</span></a>).</p> <p class="last" id="index-34">The extension modules <a class="reference internal" href="../library/dbm.html#module-dbm" title="dbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm</span></code></a>, <a class="reference internal" href="../library/gdbm.html#module-gdbm" title="gdbm: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">gdbm</span></code></a>, and <a class="reference internal" href="../library/bsddb.html#module-bsddb" title="bsddb: Interface to Berkeley DB database library"><code class="xref py py-mod docutils literal notranslate"><span class="pre">bsddb</span></code></a> provide additional examples of mapping types.</p> </dd> </dl> </dd> <dt>Callable types</dt> <dd><p class="first" id="index-35">These are the types to which the function call operation (see section <a class="reference internal" href="expressions.html#calls"><span class="std std-ref">Calls</span></a>) can be applied:</p> <dl class="last docutils"> <dt>User-defined functions</dt> <dd><p class="first" id="index-36">A user-defined function object is created by a function definition (see section <a class="reference internal" href="compound_stmts.html#function"><span class="std std-ref">Function definitions</span></a>). It should be called with an argument list containing the same number of items as the function’s formal parameter list.</p> <p>Special attributes:</p> <table border="1" class="docutils" id="index-37"> <colgroup> <col width="35%" /> <col width="48%" /> <col width="17%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Attribute</th> <th class="head">Meaning</th> <th class="head"> </th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> <code class="xref py py-attr docutils literal notranslate"><span class="pre">func_doc</span></code></td> <td>The function’s documentation string, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable.</td> <td>Writable</td> </tr> <tr class="row-odd"><td><a class="reference internal" href="../library/stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a> <code class="xref py py-attr docutils literal notranslate"><span class="pre">func_name</span></code></td> <td>The function’s name</td> <td>Writable</td> </tr> <tr class="row-even"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__module__</span></code></td> <td>The name of the module the function was defined in, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable.</td> <td>Writable</td> </tr> <tr class="row-odd"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__defaults__</span></code> <code class="xref py py-attr docutils literal notranslate"><span class="pre">func_defaults</span></code></td> <td>A tuple containing default argument values for those arguments that have defaults, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if no arguments have a default value.</td> <td>Writable</td> </tr> <tr class="row-even"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__code__</span></code> <code class="xref py py-attr docutils literal notranslate"><span class="pre">func_code</span></code></td> <td>The code object representing the compiled function body.</td> <td>Writable</td> </tr> <tr class="row-odd"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__globals__</span></code> <code class="xref py py-attr docutils literal notranslate"><span class="pre">func_globals</span></code></td> <td>A reference to the dictionary that holds the function’s global variables — the global namespace of the module in which the function was defined.</td> <td>Read-only</td> </tr> <tr class="row-even"><td><a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> <code class="xref py py-attr docutils literal notranslate"><span class="pre">func_dict</span></code></td> <td>The namespace supporting arbitrary function attributes.</td> <td>Writable</td> </tr> <tr class="row-odd"><td><code class="xref py py-attr docutils literal notranslate"><span class="pre">__closure__</span></code> <code class="xref py py-attr docutils literal notranslate"><span class="pre">func_closure</span></code></td> <td><code class="docutils literal notranslate"><span class="pre">None</span></code> or a tuple of cells that contain bindings for the function’s free variables.</td> <td>Read-only</td> </tr> </tbody> </table> <p>Most of the attributes labelled “Writable” check the type of the assigned value.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.4: </span><code class="docutils literal notranslate"><span class="pre">func_name</span></code> is now writable.</p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.6: </span>The double-underscore attributes <code class="docutils literal notranslate"><span class="pre">__closure__</span></code>, <code class="docutils literal notranslate"><span class="pre">__code__</span></code>, <code class="docutils literal notranslate"><span class="pre">__defaults__</span></code>, and <code class="docutils literal notranslate"><span class="pre">__globals__</span></code> were introduced as aliases for the corresponding <code class="docutils literal notranslate"><span class="pre">func_*</span></code> attributes for forwards compatibility with Python 3.</p> </div> <p>Function objects also support getting and setting arbitrary attributes, which can be used, for example, to attach metadata to functions. Regular attribute dot-notation is used to get and set such attributes. <em>Note that the current implementation only supports function attributes on user-defined functions. Function attributes on built-in functions may be supported in the future.</em></p> <p class="last">Additional information about a function’s definition can be retrieved from its code object; see the description of internal types below.</p> </dd> <dt>User-defined methods</dt> <dd><p class="first" id="index-38">A user-defined method object combines a class, a class instance (or <code class="docutils literal notranslate"><span class="pre">None</span></code>) and any callable object (normally a user-defined function).</p> <p>Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> is the class instance object, <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_func</span></code> is the function object; <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_class</span></code> is the class of <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> for bound methods or the class that asked for the method for unbound methods; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> is the method’s documentation (same as <code class="docutils literal notranslate"><span class="pre">im_func.__doc__</span></code>); <a class="reference internal" href="../library/stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a> is the method name (same as <code class="docutils literal notranslate"><span class="pre">im_func.__name__</span></code>); <code class="xref py py-attr docutils literal notranslate"><span class="pre">__module__</span></code> is the name of the module the method was defined in, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.2: </span><code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> used to refer to the class that defined the method.</p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.6: </span>For Python 3 forward-compatibility, <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_func</span></code> is also available as <code class="xref py py-attr docutils literal notranslate"><span class="pre">__func__</span></code>, and <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> as <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code>.</p> </div> <p id="index-39">Methods also support accessing (but not setting) the arbitrary function attributes on the underlying function object.</p> <p>User-defined method objects may be created when getting an attribute of a class (perhaps via an instance of that class), if that attribute is a user-defined function object, an unbound user-defined method object, or a class method object. When the attribute is a user-defined method object, a new method object is only created if the class from which it is being retrieved is the same as, or a derived class of, the class stored in the original method object; otherwise, the original method object is used as it is.</p> <p id="index-40">When a user-defined method object is created by retrieving a user-defined function object from a class, its <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> attribute is <code class="docutils literal notranslate"><span class="pre">None</span></code> and the method object is said to be unbound. When one is created by retrieving a user-defined function object from a class via one of its instances, its <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> attribute is the instance, and the method object is said to be bound. In either case, the new method’s <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_class</span></code> attribute is the class from which the retrieval takes place, and its <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_func</span></code> attribute is the original function object.</p> <p id="index-41">When a user-defined method object is created by retrieving another method object from a class or instance, the behaviour is the same as for a function object, except that the <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_func</span></code> attribute of the new instance is not the original method object but its <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_func</span></code> attribute.</p> <p id="index-42">When a user-defined method object is created by retrieving a class method object from a class or instance, its <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> attribute is the class itself, and its <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_func</span></code> attribute is the function object underlying the class method.</p> <p>When an unbound user-defined method object is called, the underlying function (<code class="xref py py-attr docutils literal notranslate"><span class="pre">im_func</span></code>) is called, with the restriction that the first argument must be an instance of the proper class (<code class="xref py py-attr docutils literal notranslate"><span class="pre">im_class</span></code>) or of a derived class thereof.</p> <p>When a bound user-defined method object is called, the underlying function (<code class="xref py py-attr docutils literal notranslate"><span class="pre">im_func</span></code>) is called, inserting the class instance (<code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code>) in front of the argument list. For instance, when <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code> is a class which contains a definition for a function <code class="xref py py-meth docutils literal notranslate"><span class="pre">f()</span></code>, and <code class="docutils literal notranslate"><span class="pre">x</span></code> is an instance of <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code>, calling <code class="docutils literal notranslate"><span class="pre">x.f(1)</span></code> is equivalent to calling <code class="docutils literal notranslate"><span class="pre">C.f(x,</span> <span class="pre">1)</span></code>.</p> <p>When a user-defined method object is derived from a class method object, the “class instance” stored in <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> will actually be the class itself, so that calling either <code class="docutils literal notranslate"><span class="pre">x.f(1)</span></code> or <code class="docutils literal notranslate"><span class="pre">C.f(1)</span></code> is equivalent to calling <code class="docutils literal notranslate"><span class="pre">f(C,1)</span></code> where <code class="docutils literal notranslate"><span class="pre">f</span></code> is the underlying function.</p> <p class="last">Note that the transformation from function object to (unbound or bound) method object happens each time the attribute is retrieved from the class or instance. In some cases, a fruitful optimization is to assign the attribute to a local variable and call that local variable. Also notice that this transformation only happens for user-defined functions; other callable objects (and all non-callable objects) are retrieved without transformation. It is also important to note that user-defined functions which are attributes of a class instance are not converted to bound methods; this <em>only</em> happens when the function is an attribute of the class.</p> </dd> <dt>Generator functions</dt> <dd><p class="first last" id="index-43">A function or method which uses the <a class="reference internal" href="simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement (see section <a class="reference internal" href="simple_stmts.html#yield"><span class="std std-ref">The yield statement</span></a>) is called a <em class="dfn">generator function</em>. Such a function, when called, always returns an iterator object which can be used to execute the body of the function: calling the iterator’s <a class="reference internal" href="../library/stdtypes.html#iterator.next" title="iterator.next"><code class="xref py py-meth docutils literal notranslate"><span class="pre">next()</span></code></a> method will cause the function to execute until it provides a value using the <a class="reference internal" href="simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement. When the function executes a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement or falls off the end, a <a class="reference internal" href="../library/exceptions.html#exceptions.StopIteration" title="exceptions.StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> exception is raised and the iterator will have reached the end of the set of values to be returned.</p> </dd> <dt>Built-in functions</dt> <dd><p class="first last" id="index-44">A built-in function object is a wrapper around a C function. Examples of built-in functions are <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> and <a class="reference internal" href="../library/math.html#math.sin" title="math.sin"><code class="xref py py-func docutils literal notranslate"><span class="pre">math.sin()</span></code></a> (<a class="reference internal" href="../library/math.html#module-math" title="math: Mathematical functions (sin() etc.)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">math</span></code></a> is a standard built-in module). The number and type of the arguments are determined by the C function. Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> is the function’s documentation string, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable; <a class="reference internal" href="../library/stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a> is the function’s name; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> is set to <code class="docutils literal notranslate"><span class="pre">None</span></code> (but see the next item); <code class="xref py py-attr docutils literal notranslate"><span class="pre">__module__</span></code> is the name of the module the function was defined in or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable.</p> </dd> <dt>Built-in methods</dt> <dd><p class="first last" id="index-45">This is really a different disguise of a built-in function, this time containing an object passed to the C function as an implicit extra argument. An example of a built-in method is <code class="docutils literal notranslate"><span class="pre">alist.append()</span></code>, assuming <em>alist</em> is a list object. In this case, the special read-only attribute <code class="xref py py-attr docutils literal notranslate"><span class="pre">__self__</span></code> is set to the object denoted by <em>alist</em>.</p> </dd> <dt>Class Types</dt> <dd>Class types, or “new-style classes,” are callable. These objects normally act as factories for new instances of themselves, but variations are possible for class types that override <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a>. The arguments of the call are passed to <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> and, in the typical case, to <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> to initialize the new instance.</dd> <dt>Classic Classes</dt> <dd><p class="first last" id="index-46">Class objects are described below. When a class object is called, a new class instance (also described below) is created and returned. This implies a call to the class’s <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method if it has one. Any arguments are passed on to the <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method. If there is no <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method, the class must be called without arguments.</p> </dd> <dt>Class instances</dt> <dd>Class instances are described below. Class instances are callable only when the class has a <a class="reference internal" href="#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__call__()</span></code></a> method; <code class="docutils literal notranslate"><span class="pre">x(arguments)</span></code> is a shorthand for <code class="docutils literal notranslate"><span class="pre">x.__call__(arguments)</span></code>.</dd> </dl> </dd> <dt>Modules</dt> <dd><p class="first" id="index-47">Modules are imported by the <a class="reference internal" href="simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement (see section <a class="reference internal" href="simple_stmts.html#import"><span class="std std-ref">The import statement</span></a>). A module object has a namespace implemented by a dictionary object (this is the dictionary referenced by the func_globals attribute of functions defined in the module). Attribute references are translated to lookups in this dictionary, e.g., <code class="docutils literal notranslate"><span class="pre">m.x</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">m.__dict__["x"]</span></code>. A module object does not contain the code object used to initialize the module (since it isn’t needed once the initialization is done).</p> <p>Attribute assignment updates the module’s namespace dictionary, e.g., <code class="docutils literal notranslate"><span class="pre">m.x</span> <span class="pre">=</span> <span class="pre">1</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">m.__dict__["x"]</span> <span class="pre">=</span> <span class="pre">1</span></code>.</p> <p id="index-48">Special read-only attribute: <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> is the module’s namespace as a dictionary object.</p> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> Because of the way CPython clears module dictionaries, the module dictionary will be cleared when the module falls out of scope even if the dictionary still has live references. To avoid this, copy the dictionary or keep the module around while using its dictionary directly.</p> </div> <p class="last" id="index-49">Predefined (writable) attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code> is the module’s name; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> is the module’s documentation string, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if unavailable; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__file__</span></code> is the pathname of the file from which the module was loaded, if it was loaded from a file. The <code class="xref py py-attr docutils literal notranslate"><span class="pre">__file__</span></code> attribute is not present for C modules that are statically linked into the interpreter; for extension modules loaded dynamically from a shared library, it is the pathname of the shared library file.</p> </dd> <dt>Classes</dt> <dd><p class="first">Both class types (new-style classes) and class objects (old-style/classic classes) are typically created by class definitions (see section <a class="reference internal" href="compound_stmts.html#class"><span class="std std-ref">Class definitions</span></a>). A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g., <code class="docutils literal notranslate"><span class="pre">C.x</span></code> is translated to <code class="docutils literal notranslate"><span class="pre">C.__dict__["x"]</span></code> (although for new-style classes in particular there are a number of hooks which allow for other means of locating attributes). When the attribute name is not found there, the attribute search continues in the base classes. For old-style classes, the search is depth-first, left-to-right in the order of occurrence in the base class list. New-style classes use the more complex C3 method resolution order which behaves correctly even in the presence of ‘diamond’ inheritance structures where there are multiple inheritance paths leading back to a common ancestor. Additional details on the C3 MRO used by new-style classes can be found in the documentation accompanying the 2.3 release at <a class="reference external" href="https://www.python.org/download/releases/2.3/mro/">https://www.python.org/download/releases/2.3/mro/</a>.</p> <p id="index-50">When a class attribute reference (for class <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code>, say) would yield a user-defined function object or an unbound user-defined method object whose associated class is either <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code> or one of its base classes, it is transformed into an unbound user-defined method object whose <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_class</span></code> attribute is <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code>. When it would yield a class method object, it is transformed into a bound user-defined method object whose <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> attribute is <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code>. When it would yield a static method object, it is transformed into the object wrapped by the static method object. See section <a class="reference internal" href="#descriptors"><span class="std std-ref">Implementing Descriptors</span></a> for another way in which attributes retrieved from a class may differ from those actually contained in its <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> (note that only new-style classes support descriptors).</p> <p id="index-51">Class attribute assignments update the class’s dictionary, never the dictionary of a base class.</p> <p id="index-52">A class object can be called (see above) to yield a class instance (see below).</p> <p class="last" id="index-53">Special attributes: <a class="reference internal" href="../library/stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a> is the class name; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__module__</span></code> is the module name in which the class was defined; <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> is the dictionary containing the class’s namespace; <a class="reference internal" href="../library/stdtypes.html#class.__bases__" title="class.__bases__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__bases__</span></code></a> is a tuple (possibly empty or a singleton) containing the base classes, in the order of their occurrence in the base class list; <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> is the class’s documentation string, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if undefined.</p> </dd> <dt>Class instances</dt> <dd><p class="first" id="index-54">A class instance is created by calling a class object (see above). A class instance has a namespace implemented as a dictionary which is the first place in which attribute references are searched. When an attribute is not found there, and the instance’s class has an attribute by that name, the search continues with the class attributes. If a class attribute is found that is a user-defined function object or an unbound user-defined method object whose associated class is the class (call it <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code>) of the instance for which the attribute reference was initiated or one of its bases, it is transformed into a bound user-defined method object whose <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_class</span></code> attribute is <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code> and whose <code class="xref py py-attr docutils literal notranslate"><span class="pre">im_self</span></code> attribute is the instance. Static method and class method objects are also transformed, as if they had been retrieved from class <code class="xref py py-class docutils literal notranslate"><span class="pre">C</span></code>; see above under “Classes”. See section <a class="reference internal" href="#descriptors"><span class="std std-ref">Implementing Descriptors</span></a> for another way in which attributes of a class retrieved via its instances may differ from the objects actually stored in the class’s <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a>. If no class attribute is found, and the object’s class has a <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> method, that is called to satisfy the lookup.</p> <p id="index-55">Attribute assignments and deletions update the instance’s dictionary, never a class’s dictionary. If the class has a <a class="reference internal" href="#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> or <a class="reference internal" href="#object.__delattr__" title="object.__delattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delattr__()</span></code></a> method, this is called instead of updating the instance dictionary directly.</p> <p id="index-56">Class instances can pretend to be numbers, sequences, or mappings if they have methods with certain special names. See section <a class="reference internal" href="#specialnames"><span class="std std-ref">Special method names</span></a>.</p> <p class="last" id="index-57">Special attributes: <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> is the attribute dictionary; <a class="reference internal" href="../library/stdtypes.html#instance.__class__" title="instance.__class__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__class__</span></code></a> is the instance’s class.</p> </dd> <dt>Files</dt> <dd><p class="first last" id="index-58">A file object represents an open file. File objects are created by the <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> built-in function, and also by <a class="reference internal" href="../library/os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.popen()</span></code></a>, <a class="reference internal" href="../library/os.html#os.fdopen" title="os.fdopen"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.fdopen()</span></code></a>, and the <code class="xref py py-meth docutils literal notranslate"><span class="pre">makefile()</span></code> method of socket objects (and perhaps by other functions or methods provided by extension modules). The objects <code class="docutils literal notranslate"><span class="pre">sys.stdin</span></code>, <code class="docutils literal notranslate"><span class="pre">sys.stdout</span></code> and <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code> are initialized to file objects corresponding to the interpreter’s standard input, output and error streams. See <a class="reference internal" href="../library/stdtypes.html#bltin-file-objects"><span class="std std-ref">File Objects</span></a> for complete documentation of file objects.</p> </dd> <dt>Internal types</dt> <dd><p class="first" id="index-59">A few types used internally by the interpreter are exposed to the user. Their definitions may change with future versions of the interpreter, but they are mentioned here for completeness.</p> <dl class="docutils" id="index-60"> <dt>Code objects</dt> <dd><p class="first">Code objects represent <em>byte-compiled</em> executable Python code, or <a class="reference internal" href="../glossary.html#term-bytecode"><span class="xref std std-term">bytecode</span></a>. The difference between a code object and a function object is that the function object contains an explicit reference to the function’s globals (the module in which it was defined), while a code object contains no context; also the default argument values are stored in the function object, not in the code object (because they represent values calculated at run-time). Unlike function objects, code objects are immutable and contain no references (directly or indirectly) to mutable objects.</p> <p id="index-61">Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_name</span></code> gives the function name; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_argcount</span></code> is the number of positional arguments (including arguments with default values); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_nlocals</span></code> is the number of local variables used by the function (including arguments); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_varnames</span></code> is a tuple containing the names of the local variables (starting with the argument names); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_cellvars</span></code> is a tuple containing the names of local variables that are referenced by nested functions; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_freevars</span></code> is a tuple containing the names of free variables; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_code</span></code> is a string representing the sequence of bytecode instructions; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_consts</span></code> is a tuple containing the literals used by the bytecode; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_names</span></code> is a tuple containing the names used by the bytecode; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_filename</span></code> is the filename from which the code was compiled; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_firstlineno</span></code> is the first line number of the function; <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_lnotab</span></code> is a string encoding the mapping from bytecode offsets to line numbers (for details see the source code of the interpreter); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_stacksize</span></code> is the required stack size (including local variables); <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_flags</span></code> is an integer encoding a number of flags for the interpreter.</p> <p id="index-62">The following flag bits are defined for <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_flags</span></code>: bit <code class="docutils literal notranslate"><span class="pre">0x04</span></code> is set if the function uses the <code class="docutils literal notranslate"><span class="pre">*arguments</span></code> syntax to accept an arbitrary number of positional arguments; bit <code class="docutils literal notranslate"><span class="pre">0x08</span></code> is set if the function uses the <code class="docutils literal notranslate"><span class="pre">**keywords</span></code> syntax to accept arbitrary keyword arguments; bit <code class="docutils literal notranslate"><span class="pre">0x20</span></code> is set if the function is a generator.</p> <p>Future feature declarations (<code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span> <span class="pre">division</span></code>) also use bits in <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_flags</span></code> to indicate whether a code object was compiled with a particular feature enabled: bit <code class="docutils literal notranslate"><span class="pre">0x2000</span></code> is set if the function was compiled with future division enabled; bits <code class="docutils literal notranslate"><span class="pre">0x10</span></code> and <code class="docutils literal notranslate"><span class="pre">0x1000</span></code> were used in earlier versions of Python.</p> <p>Other bits in <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_flags</span></code> are reserved for internal use.</p> <p class="last" id="index-63">If a code object represents a function, the first item in <code class="xref py py-attr docutils literal notranslate"><span class="pre">co_consts</span></code> is the documentation string of the function, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if undefined.</p> </dd> </dl> <dl class="last docutils" id="frame-objects"> <dt>Frame objects</dt> <dd><p class="first" id="index-64">Frame objects represent execution frames. They may occur in traceback objects (see below).</p> <p id="index-65">Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_back</span></code> is to the previous stack frame (towards the caller), or <code class="docutils literal notranslate"><span class="pre">None</span></code> if this is the bottom stack frame; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_code</span></code> is the code object being executed in this frame; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_locals</span></code> is the dictionary used to look up local variables; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_globals</span></code> is used for global variables; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_builtins</span></code> is used for built-in (intrinsic) names; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_restricted</span></code> is a flag indicating whether the function is executing in restricted execution mode; <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_lasti</span></code> gives the precise instruction (this is an index into the bytecode string of the code object).</p> <p class="last" id="index-66">Special writable attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_trace</span></code>, if not <code class="docutils literal notranslate"><span class="pre">None</span></code>, is a function called at the start of each source code line (this is used by the debugger); <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_exc_type</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_exc_value</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_exc_traceback</span></code> represent the last exception raised in the parent frame provided another exception was ever raised in the current frame (in all other cases they are <code class="docutils literal notranslate"><span class="pre">None</span></code>); <code class="xref py py-attr docutils literal notranslate"><span class="pre">f_lineno</span></code> is the current line number of the frame — writing to this from within a trace function jumps to the given line (only for the bottom-most frame). A debugger can implement a Jump command (aka Set Next Statement) by writing to f_lineno.</p> </dd> <dt>Traceback objects</dt> <dd><p class="first" id="index-67">Traceback objects represent a stack trace of an exception. A traceback object is created when an exception occurs. When the search for an exception handler unwinds the execution stack, at each unwound level a traceback object is inserted in front of the current traceback. When an exception handler is entered, the stack trace is made available to the program. (See section <a class="reference internal" href="compound_stmts.html#try"><span class="std std-ref">The try statement</span></a>.) It is accessible as <code class="docutils literal notranslate"><span class="pre">sys.exc_traceback</span></code>, and also as the third item of the tuple returned by <code class="docutils literal notranslate"><span class="pre">sys.exc_info()</span></code>. The latter is the preferred interface, since it works correctly when the program is using multiple threads. When the program contains no suitable handler, the stack trace is written (nicely formatted) to the standard error stream; if the interpreter is interactive, it is also made available to the user as <code class="docutils literal notranslate"><span class="pre">sys.last_traceback</span></code>.</p> <p class="last" id="index-68">Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">tb_next</span></code> is the next level in the stack trace (towards the frame where the exception occurred), or <code class="docutils literal notranslate"><span class="pre">None</span></code> if there is no next level; <code class="xref py py-attr docutils literal notranslate"><span class="pre">tb_frame</span></code> points to the execution frame of the current level; <code class="xref py py-attr docutils literal notranslate"><span class="pre">tb_lineno</span></code> gives the line number where the exception occurred; <code class="xref py py-attr docutils literal notranslate"><span class="pre">tb_lasti</span></code> indicates the precise instruction. The line number and last instruction in the traceback may differ from the line number of its frame object if the exception occurred in a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement with no matching except clause or with a finally clause.</p> </dd> <dt>Slice objects</dt> <dd><p class="first" id="index-69">Slice objects are used to represent slices when <em>extended slice syntax</em> is used. This is a slice using two colons, or multiple slices or ellipses separated by commas, e.g., <code class="docutils literal notranslate"><span class="pre">a[i:j:step]</span></code>, <code class="docutils literal notranslate"><span class="pre">a[i:j,</span> <span class="pre">k:l]</span></code>, or <code class="docutils literal notranslate"><span class="pre">a[...,</span> <span class="pre">i:j]</span></code>. They are also created by the built-in <a class="reference internal" href="../library/functions.html#slice" title="slice"><code class="xref py py-func docutils literal notranslate"><span class="pre">slice()</span></code></a> function.</p> <p id="index-70">Special read-only attributes: <code class="xref py py-attr docutils literal notranslate"><span class="pre">start</span></code> is the lower bound; <code class="xref py py-attr docutils literal notranslate"><span class="pre">stop</span></code> is the upper bound; <code class="xref py py-attr docutils literal notranslate"><span class="pre">step</span></code> is the step value; each is <code class="docutils literal notranslate"><span class="pre">None</span></code> if omitted. These attributes can have any type.</p> <p>Slice objects support one method:</p> <dl class="last method"> <dt id="slice.indices"> <code class="descclassname">slice.</code><code class="descname">indices</code><span class="sig-paren">(</span><em>self</em>, <em>length</em><span class="sig-paren">)</span><a class="headerlink" href="#slice.indices" title="Permalink to this definition">¶</a></dt> <dd><p>This method takes a single integer argument <em>length</em> and computes information about the extended slice that the slice object would describe if applied to a sequence of <em>length</em> items. It returns a tuple of three integers; respectively these are the <em>start</em> and <em>stop</em> indices and the <em>step</em> or stride length of the slice. Missing or out-of-bounds indices are handled in a manner consistent with regular slices.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.3.</span></p> </div> </dd></dl> </dd> <dt>Static method objects</dt> <dd>Static method objects provide a way of defeating the transformation of function objects to method objects described above. A static method object is a wrapper around any other object, usually a user-defined method object. When a static method object is retrieved from a class or a class instance, the object actually returned is the wrapped object, which is not subject to any further transformation. Static method objects are not themselves callable, although the objects they wrap usually are. Static method objects are created by the built-in <a class="reference internal" href="../library/functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">staticmethod()</span></code></a> constructor.</dd> <dt>Class method objects</dt> <dd>A class method object, like a static method object, is a wrapper around another object that alters the way in which that object is retrieved from classes and class instances. The behaviour of class method objects upon such retrieval is described above, under “User-defined methods”. Class method objects are created by the built-in <a class="reference internal" href="../library/functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">classmethod()</span></code></a> constructor.</dd> </dl> </dd> </dl> </div> <div class="section" id="new-style-and-classic-classes"> <span id="newstyle"></span><h2>3.3. New-style and classic classes<a class="headerlink" href="#new-style-and-classic-classes" title="Permalink to this headline">¶</a></h2> <p>Classes and instances come in two flavors: old-style (or classic) and new-style.</p> <p>Up to Python 2.1 the concept of <code class="docutils literal notranslate"><span class="pre">class</span></code> was unrelated to the concept of <code class="docutils literal notranslate"><span class="pre">type</span></code>, and old-style classes were the only flavor available. For an old-style class, the statement <code class="docutils literal notranslate"><span class="pre">x.__class__</span></code> provides the class of <em>x</em>, but <code class="docutils literal notranslate"><span class="pre">type(x)</span></code> is always <code class="docutils literal notranslate"><span class="pre"><type</span> <span class="pre">'instance'></span></code>. This reflects the fact that all old-style instances, independent of their class, are implemented with a single built-in type, called <code class="docutils literal notranslate"><span class="pre">instance</span></code>.</p> <p>New-style classes were introduced in Python 2.2 to unify the concepts of <code class="docutils literal notranslate"><span class="pre">class</span></code> and <code class="docutils literal notranslate"><span class="pre">type</span></code>. A new-style class is simply a user-defined type, no more, no less. If <em>x</em> is an instance of a new-style class, then <code class="docutils literal notranslate"><span class="pre">type(x)</span></code> is typically the same as <code class="docutils literal notranslate"><span class="pre">x.__class__</span></code> (although this is not guaranteed – a new-style class instance is permitted to override the value returned for <code class="docutils literal notranslate"><span class="pre">x.__class__</span></code>).</p> <p>The major motivation for introducing new-style classes is to provide a unified object model with a full meta-model. It also has a number of practical benefits, like the ability to subclass most built-in types, or the introduction of “descriptors”, which enable computed properties.</p> <p>For compatibility reasons, classes are still old-style by default. New-style classes are created by specifying another new-style class (i.e. a type) as a parent class, or the “top-level type” <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a> if no other parent is needed. The behaviour of new-style classes differs from that of old-style classes in a number of important details in addition to what <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a> returns. Some of these changes are fundamental to the new object model, like the way special methods are invoked. Others are “fixes” that could not be implemented before for compatibility concerns, like the method resolution order in case of multiple inheritance.</p> <p>While this manual aims to provide comprehensive coverage of Python’s class mechanics, it may still be lacking in some areas when it comes to its coverage of new-style classes. Please see <a class="reference external" href="https://www.python.org/doc/newstyle/">https://www.python.org/doc/newstyle/</a> for sources of additional information.</p> <p id="index-71">Old-style classes are removed in Python 3, leaving only new-style classes.</p> </div> <div class="section" id="special-method-names"> <span id="specialnames"></span><h2>3.4. Special method names<a class="headerlink" href="#special-method-names" title="Permalink to this headline">¶</a></h2> <p id="index-72">A class can implement certain operations that are invoked by special syntax (such as arithmetic operations or subscripting and slicing) by defining methods with special names. This is Python’s approach to <em class="dfn">operator overloading</em>, allowing classes to define their own behavior with respect to language operators. For instance, if a class defines a method named <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, and <code class="docutils literal notranslate"><span class="pre">x</span></code> is an instance of this class, then <code class="docutils literal notranslate"><span class="pre">x[i]</span></code> is roughly equivalent to <code class="docutils literal notranslate"><span class="pre">x.__getitem__(i)</span></code> for old-style classes and <code class="docutils literal notranslate"><span class="pre">type(x).__getitem__(x,</span> <span class="pre">i)</span></code> for new-style classes. Except where mentioned, attempts to execute an operation raise an exception when no appropriate method is defined (typically <a class="reference internal" href="../library/exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> or <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>).</p> <p>When implementing a class that emulates any built-in type, it is important that the emulation only be implemented to the degree that it makes sense for the object being modelled. For example, some sequences may work well with retrieval of individual elements, but extracting a slice may not make sense. (One example of this is the <code class="xref py py-class docutils literal notranslate"><span class="pre">NodeList</span></code> interface in the W3C’s Document Object Model.)</p> <div class="section" id="basic-customization"> <span id="customization"></span><h3>3.4.1. Basic customization<a class="headerlink" href="#basic-customization" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="object.__new__"> <code class="descclassname">object.</code><code class="descname">__new__</code><span class="sig-paren">(</span><em>cls</em><span class="optional">[</span>, <em>...</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__new__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-73">Called to create a new instance of class <em>cls</em>. <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> is a static method (special-cased so you need not declare it as such) that takes the class of which an instance was requested as its first argument. The remaining arguments are those passed to the object constructor expression (the call to the class). The return value of <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> should be the new object instance (usually an instance of <em>cls</em>).</p> <p>Typical implementations create a new instance of the class by invoking the superclass’s <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> method using <code class="docutils literal notranslate"><span class="pre">super(currentclass,</span> <span class="pre">cls).__new__(cls[,</span> <span class="pre">...])</span></code> with appropriate arguments and then modifying the newly-created instance as necessary before returning it.</p> <p>If <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> returns an instance of <em>cls</em>, then the new instance’s <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method will be invoked like <code class="docutils literal notranslate"><span class="pre">__init__(self[,</span> <span class="pre">...])</span></code>, where <em>self</em> is the new instance and the remaining arguments are the same as were passed to <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a>.</p> <p>If <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> does not return an instance of <em>cls</em>, then the new instance’s <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method will not be invoked.</p> <p><a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> is intended mainly to allow subclasses of immutable types (like int, str, or tuple) to customize instance creation. It is also commonly overridden in custom metaclasses in order to customize class creation.</p> </dd></dl> <dl class="method"> <dt id="object.__init__"> <code class="descclassname">object.</code><code class="descname">__init__</code><span class="sig-paren">(</span><em>self</em><span class="optional">[</span>, <em>...</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__init__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-74">Called after the instance has been created (by <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a>), but before it is returned to the caller. The arguments are those passed to the class constructor expression. If a base class has an <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method, the derived class’s <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: <code class="docutils literal notranslate"><span class="pre">BaseClass.__init__(self,</span> <span class="pre">[args...])</span></code>.</p> <p>Because <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> and <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> work together in constructing objects (<a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> to create it, and <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> to customise it), no non-<code class="docutils literal notranslate"><span class="pre">None</span></code> value may be returned by <a class="reference internal" href="#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a>; doing so will cause a <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> to be raised at runtime.</p> </dd></dl> <dl class="method"> <dt id="object.__del__"> <code class="descclassname">object.</code><code class="descname">__del__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__del__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-75">Called when the instance is about to be destroyed. This is also called a destructor. If a base class has a <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method, the derived class’s <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method, if any, must explicitly call it to ensure proper deletion of the base class part of the instance. Note that it is possible (though not recommended!) for the <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method to postpone destruction of the instance by creating a new reference to it. It may then be called at a later time when this new reference is deleted. It is not guaranteed that <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods are called for objects that still exist when the interpreter exits.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last"><code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">x</span></code> doesn’t directly call <code class="docutils literal notranslate"><span class="pre">x.__del__()</span></code> — the former decrements the reference count for <code class="docutils literal notranslate"><span class="pre">x</span></code> by one, and the latter is only called when <code class="docutils literal notranslate"><span class="pre">x</span></code>’s reference count reaches zero. Some common situations that may prevent the reference count of an object from going to zero include: circular references between objects (e.g., a doubly-linked list or a tree data structure with parent and child pointers); a reference to the object on the stack frame of a function that caught an exception (the traceback stored in <code class="docutils literal notranslate"><span class="pre">sys.exc_traceback</span></code> keeps the stack frame alive); or a reference to the object on the stack frame that raised an unhandled exception in interactive mode (the traceback stored in <code class="docutils literal notranslate"><span class="pre">sys.last_traceback</span></code> keeps the stack frame alive). The first situation can only be remedied by explicitly breaking the cycles; the latter two situations can be resolved by storing <code class="docutils literal notranslate"><span class="pre">None</span></code> in <code class="docutils literal notranslate"><span class="pre">sys.exc_traceback</span></code> or <code class="docutils literal notranslate"><span class="pre">sys.last_traceback</span></code>. Circular references which are garbage are detected when the option cycle detector is enabled (it’s on by default), but can only be cleaned up if there are no Python-level <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods involved. Refer to the documentation for the <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module for more information about how <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods are handled by the cycle detector, particularly the description of the <code class="docutils literal notranslate"><span class="pre">garbage</span></code> value.</p> </div> <div class="admonition warning"> <p class="first admonition-title">Warning</p> <p class="last">Due to the precarious circumstances under which <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods are invoked, exceptions that occur during their execution are ignored, and a warning is printed to <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code> instead. Also, when <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> is invoked in response to a module being deleted (e.g., when execution of the program is done), other globals referenced by the <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method may already have been deleted or in the process of being torn down (e.g. the import machinery shutting down). For this reason, <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods should do the absolute minimum needed to maintain external invariants. Starting with version 1.5, Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted; if no other references to such globals exist, this may help in assuring that imported modules are still available at the time when the <a class="reference internal" href="#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method is called.</p> </div> <p>See also the <a class="reference internal" href="../using/cmdline.html#cmdoption-r"><code class="xref std std-option docutils literal notranslate"><span class="pre">-R</span></code></a> command-line option.</p> </dd></dl> <dl class="method"> <dt id="object.__repr__"> <code class="descclassname">object.</code><code class="descname">__repr__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__repr__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-76">Called by the <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate"><span class="pre">repr()</span></code></a> built-in function and by string conversions (reverse quotes) to compute the “official” string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <code class="docutils literal notranslate"><span class="pre"><...some</span> <span class="pre">useful</span> <span class="pre">description...></span></code> should be returned. The return value must be a string object. If a class defines <a class="reference internal" href="#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> but not <a class="reference internal" href="#object.__str__" title="object.__str__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__str__()</span></code></a>, then <a class="reference internal" href="#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> is also used when an “informal” string representation of instances of that class is required.</p> <p id="index-77">This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.</p> </dd></dl> <dl class="method"> <dt id="object.__str__"> <code class="descclassname">object.</code><code class="descname">__str__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__str__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-78">Called by the <a class="reference internal" href="../library/functions.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str()</span></code></a> built-in function and by the <a class="reference internal" href="simple_stmts.html#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a> statement to compute the “informal” string representation of an object. This differs from <a class="reference internal" href="#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> in that it does not have to be a valid Python expression: a more convenient or concise representation may be used instead. The return value must be a string object.</p> </dd></dl> <dl class="method"> <dt id="object.__lt__"> <code class="descclassname">object.</code><code class="descname">__lt__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__lt__" title="Permalink to this definition">¶</a></dt> <dt id="object.__le__"> <code class="descclassname">object.</code><code class="descname">__le__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__le__" title="Permalink to this definition">¶</a></dt> <dt id="object.__eq__"> <code class="descclassname">object.</code><code class="descname">__eq__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__eq__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ne__"> <code class="descclassname">object.</code><code class="descname">__ne__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ne__" title="Permalink to this definition">¶</a></dt> <dt id="object.__gt__"> <code class="descclassname">object.</code><code class="descname">__gt__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__gt__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ge__"> <code class="descclassname">object.</code><code class="descname">__ge__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ge__" title="Permalink to this definition">¶</a></dt> <dd><div class="versionadded"> <p><span class="versionmodified">New in version 2.1.</span></p> </div> <p id="index-79">These are the so-called “rich comparison” methods, and are called for comparison operators in preference to <a class="reference internal" href="#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a> below. The correspondence between operator symbols and method names is as follows: <code class="docutils literal notranslate"><span class="pre">x<y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__lt__(y)</span></code>, <code class="docutils literal notranslate"><span class="pre">x<=y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__le__(y)</span></code>, <code class="docutils literal notranslate"><span class="pre">x==y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__eq__(y)</span></code>, <code class="docutils literal notranslate"><span class="pre">x!=y</span></code> and <code class="docutils literal notranslate"><span class="pre">x<>y</span></code> call <code class="docutils literal notranslate"><span class="pre">x.__ne__(y)</span></code>, <code class="docutils literal notranslate"><span class="pre">x>y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__gt__(y)</span></code>, and <code class="docutils literal notranslate"><span class="pre">x>=y</span></code> calls <code class="docutils literal notranslate"><span class="pre">x.__ge__(y)</span></code>.</p> <p>A rich comparison method may return the singleton <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> if it does not implement the operation for a given pair of arguments. By convention, <code class="docutils literal notranslate"><span class="pre">False</span></code> and <code class="docutils literal notranslate"><span class="pre">True</span></code> are returned for a successful comparison. However, these methods can return any value, so if the comparison operator is used in a Boolean context (e.g., in the condition of an <code class="docutils literal notranslate"><span class="pre">if</span></code> statement), Python will call <a class="reference internal" href="../library/functions.html#bool" title="bool"><code class="xref py py-func docutils literal notranslate"><span class="pre">bool()</span></code></a> on the value to determine if the result is true or false.</p> <p>There are no implied relationships among the comparison operators. The truth of <code class="docutils literal notranslate"><span class="pre">x==y</span></code> does not imply that <code class="docutils literal notranslate"><span class="pre">x!=y</span></code> is false. Accordingly, when defining <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a>, one should also define <a class="reference internal" href="#object.__ne__" title="object.__ne__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ne__()</span></code></a> so that the operators will behave as expected. See the paragraph on <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> for some important notes on creating <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a> objects which support custom comparison operations and are usable as dictionary keys.</p> <p>There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, <a class="reference internal" href="#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a> and <a class="reference internal" href="#object.__gt__" title="object.__gt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__gt__()</span></code></a> are each other’s reflection, <a class="reference internal" href="#object.__le__" title="object.__le__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__le__()</span></code></a> and <a class="reference internal" href="#object.__ge__" title="object.__ge__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ge__()</span></code></a> are each other’s reflection, and <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> and <a class="reference internal" href="#object.__ne__" title="object.__ne__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ne__()</span></code></a> are their own reflection.</p> <p>Arguments to rich comparison methods are never coerced.</p> <p>To automatically generate ordering operations from a single root operation, see <a class="reference internal" href="../library/functools.html#functools.total_ordering" title="functools.total_ordering"><code class="xref py py-func docutils literal notranslate"><span class="pre">functools.total_ordering()</span></code></a>.</p> </dd></dl> <dl class="method"> <dt id="object.__cmp__"> <code class="descclassname">object.</code><code class="descname">__cmp__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__cmp__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-80">Called by comparison operations if rich comparison (see above) is not defined. Should return a negative integer if <code class="docutils literal notranslate"><span class="pre">self</span> <span class="pre"><</span> <span class="pre">other</span></code>, zero if <code class="docutils literal notranslate"><span class="pre">self</span> <span class="pre">==</span> <span class="pre">other</span></code>, a positive integer if <code class="docutils literal notranslate"><span class="pre">self</span> <span class="pre">></span> <span class="pre">other</span></code>. If no <a class="reference internal" href="#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a>, <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> or <a class="reference internal" href="#object.__ne__" title="object.__ne__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ne__()</span></code></a> operation is defined, class instances are compared by object identity (“address”). See also the description of <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> for some important notes on creating <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a> objects which support custom comparison operations and are usable as dictionary keys. (Note: the restriction that exceptions are not propagated by <a class="reference internal" href="#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a> has been removed since Python 1.5.)</p> </dd></dl> <dl class="method"> <dt id="object.__rcmp__"> <code class="descclassname">object.</code><code class="descname">__rcmp__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rcmp__" title="Permalink to this definition">¶</a></dt> <dd><div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.1: </span>No longer supported.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__hash__"> <code class="descclassname">object.</code><code class="descname">__hash__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__hash__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-81">Called by built-in function <a class="reference internal" href="../library/functions.html#hash" title="hash"><code class="xref py py-func docutils literal notranslate"><span class="pre">hash()</span></code></a> and for operations on members of hashed collections including <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate"><span class="pre">set</span></code></a>, <a class="reference internal" href="../library/stdtypes.html#frozenset" title="frozenset"><code class="xref py py-class docutils literal notranslate"><span class="pre">frozenset</span></code></a>, and <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a>. <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> should return an integer. The only required property is that objects which compare equal have the same hash value; it is advised to mix together the hash values of the components of the object that also play a part in comparison of objects by packing them into a tuple and hashing the tuple. Example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">__hash__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="nb">hash</span><span class="p">((</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">nick</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">color</span><span class="p">))</span> </pre></div> </div> <p>If a class does not define a <a class="reference internal" href="#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a> or <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> method it should not define a <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> operation either; if it defines <a class="reference internal" href="#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a> or <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> but not <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a>, its instances will not be usable in hashed collections. If a class defines mutable objects and implements a <a class="reference internal" href="#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a> or <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> method, it should not implement <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a>, since hashable collection implementations require that an object’s hash value is immutable (if the object’s hash value changes, it will be in the wrong hash bucket).</p> <p>User-defined classes have <a class="reference internal" href="#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a> and <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> methods by default; with them, all objects compare unequal (except with themselves) and <code class="docutils literal notranslate"><span class="pre">x.__hash__()</span></code> returns a result derived from <code class="docutils literal notranslate"><span class="pre">id(x)</span></code>.</p> <p>Classes which inherit a <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method from a parent class but change the meaning of <a class="reference internal" href="#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a> or <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> such that the hash value returned is no longer appropriate (e.g. by switching to a value-based concept of equality instead of the default identity based equality) can explicitly flag themselves as being unhashable by setting <code class="docutils literal notranslate"><span class="pre">__hash__</span> <span class="pre">=</span> <span class="pre">None</span></code> in the class definition. Doing so means that not only will instances of the class raise an appropriate <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> when a program attempts to retrieve their hash value, but they will also be correctly identified as unhashable when checking <code class="docutils literal notranslate"><span class="pre">isinstance(obj,</span> <span class="pre">collections.Hashable)</span></code> (unlike classes which define their own <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> to explicitly raise <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>).</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.5: </span><a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> may now also return a long integer object; the 32-bit integer is then derived from the hash of that object.</p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.6: </span><a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__hash__</span></code></a> may now be set to <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a> to explicitly flag instances of a class as unhashable.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__nonzero__"> <code class="descclassname">object.</code><code class="descname">__nonzero__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__nonzero__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-82">Called to implement truth value testing and the built-in operation <code class="docutils literal notranslate"><span class="pre">bool()</span></code>; should return <code class="docutils literal notranslate"><span class="pre">False</span></code> or <code class="docutils literal notranslate"><span class="pre">True</span></code>, or their integer equivalents <code class="docutils literal notranslate"><span class="pre">0</span></code> or <code class="docutils literal notranslate"><span class="pre">1</span></code>. When this method is not defined, <a class="reference internal" href="#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> is called, if it is defined, and the object is considered true if its result is nonzero. If a class defines neither <a class="reference internal" href="#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> nor <a class="reference internal" href="#object.__nonzero__" title="object.__nonzero__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__nonzero__()</span></code></a>, all its instances are considered true.</p> </dd></dl> <dl class="method"> <dt id="object.__unicode__"> <code class="descclassname">object.</code><code class="descname">__unicode__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__unicode__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-83">Called to implement <a class="reference internal" href="../library/functions.html#unicode" title="unicode"><code class="xref py py-func docutils literal notranslate"><span class="pre">unicode()</span></code></a> built-in; should return a Unicode object. When this method is not defined, string conversion is attempted, and the result of string conversion is converted to Unicode using the system default encoding.</p> </dd></dl> </div> <div class="section" id="customizing-attribute-access"> <span id="attribute-access"></span><h3>3.4.2. Customizing attribute access<a class="headerlink" href="#customizing-attribute-access" title="Permalink to this headline">¶</a></h3> <p>The following methods can be defined to customize the meaning of attribute access (use of, assignment to, or deletion of <code class="docutils literal notranslate"><span class="pre">x.name</span></code>) for class instances.</p> <dl class="method"> <dt id="object.__getattr__"> <code class="descclassname">object.</code><code class="descname">__getattr__</code><span class="sig-paren">(</span><em>self</em>, <em>name</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__getattr__" title="Permalink to this definition">¶</a></dt> <dd><p>Called when an attribute lookup has not found the attribute in the usual places (i.e. it is not an instance attribute nor is it found in the class tree for <code class="docutils literal notranslate"><span class="pre">self</span></code>). <code class="docutils literal notranslate"><span class="pre">name</span></code> is the attribute name. This method should return the (computed) attribute value or raise an <a class="reference internal" href="../library/exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> exception.</p> <p id="index-84">Note that if the attribute is found through the normal mechanism, <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> is not called. (This is an intentional asymmetry between <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> and <a class="reference internal" href="#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a>.) This is done both for efficiency reasons and because otherwise <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a> would have no way to access other attributes of the instance. Note that at least for instance variables, you can fake total control by not inserting any values in the instance attribute dictionary (but instead inserting them in another object). See the <a class="reference internal" href="#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> method below for a way to actually get total control in new-style classes.</p> </dd></dl> <dl class="method"> <dt id="object.__setattr__"> <code class="descclassname">object.</code><code class="descname">__setattr__</code><span class="sig-paren">(</span><em>self</em>, <em>name</em>, <em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__setattr__" title="Permalink to this definition">¶</a></dt> <dd><p>Called when an attribute assignment is attempted. This is called instead of the normal mechanism (i.e. store the value in the instance dictionary). <em>name</em> is the attribute name, <em>value</em> is the value to be assigned to it.</p> <p id="index-85">If <a class="reference internal" href="#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> wants to assign to an instance attribute, it should not simply execute <code class="docutils literal notranslate"><span class="pre">self.name</span> <span class="pre">=</span> <span class="pre">value</span></code> — this would cause a recursive call to itself. Instead, it should insert the value in the dictionary of instance attributes, e.g., <code class="docutils literal notranslate"><span class="pre">self.__dict__[name]</span> <span class="pre">=</span> <span class="pre">value</span></code>. For new-style classes, rather than accessing the instance dictionary, it should call the base class method with the same name, for example, <code class="docutils literal notranslate"><span class="pre">object.__setattr__(self,</span> <span class="pre">name,</span> <span class="pre">value)</span></code>.</p> </dd></dl> <dl class="method"> <dt id="object.__delattr__"> <code class="descclassname">object.</code><code class="descname">__delattr__</code><span class="sig-paren">(</span><em>self</em>, <em>name</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__delattr__" title="Permalink to this definition">¶</a></dt> <dd><p>Like <a class="reference internal" href="#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> but for attribute deletion instead of assignment. This should only be implemented if <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">obj.name</span></code> is meaningful for the object.</p> </dd></dl> <div class="section" id="more-attribute-access-for-new-style-classes"> <span id="new-style-attribute-access"></span><h4>3.4.2.1. More attribute access for new-style classes<a class="headerlink" href="#more-attribute-access-for-new-style-classes" title="Permalink to this headline">¶</a></h4> <p>The following methods only apply to new-style classes.</p> <dl class="method"> <dt id="object.__getattribute__"> <code class="descclassname">object.</code><code class="descname">__getattribute__</code><span class="sig-paren">(</span><em>self</em>, <em>name</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__getattribute__" title="Permalink to this definition">¶</a></dt> <dd><p>Called unconditionally to implement attribute accesses for instances of the class. If the class also defines <a class="reference internal" href="#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a>, the latter will not be called unless <a class="reference internal" href="#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> either calls it explicitly or raises an <a class="reference internal" href="../library/exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a>. This method should return the (computed) attribute value or raise an <a class="reference internal" href="../library/exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> exception. In order to avoid infinite recursion in this method, its implementation should always call the base class method with the same name to access any attributes it needs, for example, <code class="docutils literal notranslate"><span class="pre">object.__getattribute__(self,</span> <span class="pre">name)</span></code>.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">This method may still be bypassed when looking up special methods as the result of implicit invocation via language syntax or built-in functions. See <a class="reference internal" href="#new-style-special-lookup"><span class="std std-ref">Special method lookup for new-style classes</span></a>.</p> </div> </dd></dl> </div> <div class="section" id="implementing-descriptors"> <span id="descriptors"></span><h4>3.4.2.2. Implementing Descriptors<a class="headerlink" href="#implementing-descriptors" title="Permalink to this headline">¶</a></h4> <p>The following methods only apply when an instance of the class containing the method (a so-called <em>descriptor</em> class) appears in an <em>owner</em> class (the descriptor must be in either the owner’s class dictionary or in the class dictionary for one of its parents). In the examples below, “the attribute” refers to the attribute whose name is the key of the property in the owner class’ <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a>.</p> <dl class="method"> <dt id="object.__get__"> <code class="descclassname">object.</code><code class="descname">__get__</code><span class="sig-paren">(</span><em>self</em>, <em>instance</em>, <em>owner</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__get__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to get the attribute of the owner class (class attribute access) or of an instance of that class (instance attribute access). <em>owner</em> is always the owner class, while <em>instance</em> is the instance that the attribute was accessed through, or <code class="docutils literal notranslate"><span class="pre">None</span></code> when the attribute is accessed through the <em>owner</em>. This method should return the (computed) attribute value or raise an <a class="reference internal" href="../library/exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> exception.</p> </dd></dl> <dl class="method"> <dt id="object.__set__"> <code class="descclassname">object.</code><code class="descname">__set__</code><span class="sig-paren">(</span><em>self</em>, <em>instance</em>, <em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__set__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to set the attribute on an instance <em>instance</em> of the owner class to a new value, <em>value</em>.</p> </dd></dl> <dl class="method"> <dt id="object.__delete__"> <code class="descclassname">object.</code><code class="descname">__delete__</code><span class="sig-paren">(</span><em>self</em>, <em>instance</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__delete__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to delete the attribute on an instance <em>instance</em> of the owner class.</p> </dd></dl> </div> <div class="section" id="invoking-descriptors"> <span id="descriptor-invocation"></span><h4>3.4.2.3. Invoking Descriptors<a class="headerlink" href="#invoking-descriptors" title="Permalink to this headline">¶</a></h4> <p>In general, a descriptor is an object attribute with “binding behavior”, one whose attribute access has been overridden by methods in the descriptor protocol: <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a>, <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a>, and <a class="reference internal" href="#object.__delete__" title="object.__delete__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delete__()</span></code></a>. If any of those methods are defined for an object, it is said to be a descriptor.</p> <p>The default behavior for attribute access is to get, set, or delete the attribute from an object’s dictionary. For instance, <code class="docutils literal notranslate"><span class="pre">a.x</span></code> has a lookup chain starting with <code class="docutils literal notranslate"><span class="pre">a.__dict__['x']</span></code>, then <code class="docutils literal notranslate"><span class="pre">type(a).__dict__['x']</span></code>, and continuing through the base classes of <code class="docutils literal notranslate"><span class="pre">type(a)</span></code> excluding metaclasses.</p> <p>However, if the looked-up value is an object defining one of the descriptor methods, then Python may override the default behavior and invoke the descriptor method instead. Where this occurs in the precedence chain depends on which descriptor methods were defined and how they were called. Note that descriptors are only invoked for new style objects or classes (ones that subclass <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object()</span></code></a> or <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-class docutils literal notranslate"><span class="pre">type()</span></code></a>).</p> <p>The starting point for descriptor invocation is a binding, <code class="docutils literal notranslate"><span class="pre">a.x</span></code>. How the arguments are assembled depends on <code class="docutils literal notranslate"><span class="pre">a</span></code>:</p> <dl class="docutils"> <dt>Direct Call</dt> <dd>The simplest and least common call is when user code directly invokes a descriptor method: <code class="docutils literal notranslate"><span class="pre">x.__get__(a)</span></code>.</dd> <dt>Instance Binding</dt> <dd>If binding to a new-style object instance, <code class="docutils literal notranslate"><span class="pre">a.x</span></code> is transformed into the call: <code class="docutils literal notranslate"><span class="pre">type(a).__dict__['x'].__get__(a,</span> <span class="pre">type(a))</span></code>.</dd> <dt>Class Binding</dt> <dd>If binding to a new-style class, <code class="docutils literal notranslate"><span class="pre">A.x</span></code> is transformed into the call: <code class="docutils literal notranslate"><span class="pre">A.__dict__['x'].__get__(None,</span> <span class="pre">A)</span></code>.</dd> <dt>Super Binding</dt> <dd>If <code class="docutils literal notranslate"><span class="pre">a</span></code> is an instance of <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-class docutils literal notranslate"><span class="pre">super</span></code></a>, then the binding <code class="docutils literal notranslate"><span class="pre">super(B,</span> <span class="pre">obj).m()</span></code> searches <code class="docutils literal notranslate"><span class="pre">obj.__class__.__mro__</span></code> for the base class <code class="docutils literal notranslate"><span class="pre">A</span></code> immediately preceding <code class="docutils literal notranslate"><span class="pre">B</span></code> and then invokes the descriptor with the call: <code class="docutils literal notranslate"><span class="pre">A.__dict__['m'].__get__(obj,</span> <span class="pre">obj.__class__)</span></code>.</dd> </dl> <p>For instance bindings, the precedence of descriptor invocation depends on the which descriptor methods are defined. A descriptor can define any combination of <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a>, <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a> and <a class="reference internal" href="#object.__delete__" title="object.__delete__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delete__()</span></code></a>. If it does not define <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a>, then accessing the attribute will return the descriptor object itself unless there is a value in the object’s instance dictionary. If the descriptor defines <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a> and/or <a class="reference internal" href="#object.__delete__" title="object.__delete__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delete__()</span></code></a>, it is a data descriptor; if it defines neither, it is a non-data descriptor. Normally, data descriptors define both <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> and <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a>, while non-data descriptors have just the <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> method. Data descriptors with <a class="reference internal" href="#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__set__()</span></code></a> and <a class="reference internal" href="#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__get__()</span></code></a> defined always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by instances.</p> <p>Python methods (including <a class="reference internal" href="../library/functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">staticmethod()</span></code></a> and <a class="reference internal" href="../library/functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate"><span class="pre">classmethod()</span></code></a>) are implemented as non-data descriptors. Accordingly, instances can redefine and override methods. This allows individual instances to acquire behaviors that differ from other instances of the same class.</p> <p>The <a class="reference internal" href="../library/functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a> function is implemented as a data descriptor. Accordingly, instances cannot override the behavior of a property.</p> </div> <div class="section" id="slots"> <span id="id2"></span><h4>3.4.2.4. __slots__<a class="headerlink" href="#slots" title="Permalink to this headline">¶</a></h4> <p>By default, instances of both old and new-style classes have a dictionary for attribute storage. This wastes space for objects having very few instance variables. The space consumption can become acute when creating large numbers of instances.</p> <p>The default can be overridden by defining <em>__slots__</em> in a new-style class definition. The <em>__slots__</em> declaration takes a sequence of instance variables and reserves just enough space in each instance to hold a value for each variable. Space is saved because <em>__dict__</em> is not created for each instance.</p> <dl class="data"> <dt id="__slots__"> <code class="descname">__slots__</code><a class="headerlink" href="#__slots__" title="Permalink to this definition">¶</a></dt> <dd><p>This class variable can be assigned a string, iterable, or sequence of strings with variable names used by instances. If defined in a new-style class, <em>__slots__</em> reserves space for the declared variables and prevents the automatic creation of <em>__dict__</em> and <em>__weakref__</em> for each instance.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.2.</span></p> </div> </dd></dl> <p>Notes on using <em>__slots__</em></p> <ul> <li><p class="first">When inheriting from a class without <em>__slots__</em>, the <em>__dict__</em> attribute of that class will always be accessible, so a <em>__slots__</em> definition in the subclass is meaningless.</p> </li> <li><p class="first">Without a <em>__dict__</em> variable, instances cannot be assigned new variables not listed in the <em>__slots__</em> definition. Attempts to assign to an unlisted variable name raises <a class="reference internal" href="../library/exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a>. If dynamic assignment of new variables is desired, then add <code class="docutils literal notranslate"><span class="pre">'__dict__'</span></code> to the sequence of strings in the <em>__slots__</em> declaration.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.3: </span>Previously, adding <code class="docutils literal notranslate"><span class="pre">'__dict__'</span></code> to the <em>__slots__</em> declaration would not enable the assignment of new attributes not specifically listed in the sequence of instance variable names.</p> </div> </li> <li><p class="first">Without a <em>__weakref__</em> variable for each instance, classes defining <em>__slots__</em> do not support weak references to its instances. If weak reference support is needed, then add <code class="docutils literal notranslate"><span class="pre">'__weakref__'</span></code> to the sequence of strings in the <em>__slots__</em> declaration.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.3: </span>Previously, adding <code class="docutils literal notranslate"><span class="pre">'__weakref__'</span></code> to the <em>__slots__</em> declaration would not enable support for weak references.</p> </div> </li> <li><p class="first"><em>__slots__</em> are implemented at the class level by creating descriptors (<a class="reference internal" href="#descriptors"><span class="std std-ref">Implementing Descriptors</span></a>) for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by <em>__slots__</em>; otherwise, the class attribute would overwrite the descriptor assignment.</p> </li> <li><p class="first">The action of a <em>__slots__</em> declaration is limited to the class where it is defined. As a result, subclasses will have a <em>__dict__</em> unless they also define <em>__slots__</em> (which must only contain names of any <em>additional</em> slots).</p> </li> <li><p class="first">If a class defines a slot also defined in a base class, the instance variable defined by the base class slot is inaccessible (except by retrieving its descriptor directly from the base class). This renders the meaning of the program undefined. In the future, a check may be added to prevent this.</p> </li> <li><p class="first">Nonempty <em>__slots__</em> does not work for classes derived from “variable-length” built-in types such as <a class="reference internal" href="../library/functions.html#long" title="long"><code class="xref py py-class docutils literal notranslate"><span class="pre">long</span></code></a>, <a class="reference internal" href="../library/functions.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a> and <a class="reference internal" href="../library/functions.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate"><span class="pre">tuple</span></code></a>.</p> </li> <li><p class="first">Any non-string iterable may be assigned to <em>__slots__</em>. Mappings may also be used; however, in the future, special meaning may be assigned to the values corresponding to each key.</p> </li> <li><p class="first"><em>__class__</em> assignment works only if both classes have the same <em>__slots__</em>.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.6: </span>Previously, <em>__class__</em> assignment raised an error if either new or old class had <em>__slots__</em>.</p> </div> </li> </ul> </div> </div> <div class="section" id="customizing-class-creation"> <span id="metaclasses"></span><h3>3.4.3. Customizing class creation<a class="headerlink" href="#customizing-class-creation" title="Permalink to this headline">¶</a></h3> <p>By default, new-style classes are constructed using <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a>. A class definition is read into a separate namespace and the value of class name is bound to the result of <code class="docutils literal notranslate"><span class="pre">type(name,</span> <span class="pre">bases,</span> <span class="pre">dict)</span></code>.</p> <p>When the class definition is read, if <em>__metaclass__</em> is defined then the callable assigned to it will be called instead of <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a>. This allows classes or functions to be written which monitor or alter the class creation process:</p> <ul class="simple"> <li>Modifying the class dictionary prior to the class being created.</li> <li>Returning an instance of another class – essentially performing the role of a factory function.</li> </ul> <p>These steps will have to be performed in the metaclass’s <a class="reference internal" href="#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> method – <code class="xref py py-meth docutils literal notranslate"><span class="pre">type.__new__()</span></code> can then be called from this method to create a class with different properties. This example adds a new element to the class dictionary before creating the class:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">metacls</span><span class="p">(</span><span class="nb">type</span><span class="p">):</span> <span class="k">def</span> <span class="nf">__new__</span><span class="p">(</span><span class="n">mcs</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">bases</span><span class="p">,</span> <span class="nb">dict</span><span class="p">):</span> <span class="nb">dict</span><span class="p">[</span><span class="s1">'foo'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'metacls was here'</span> <span class="k">return</span> <span class="nb">type</span><span class="o">.</span><span class="fm">__new__</span><span class="p">(</span><span class="n">mcs</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">bases</span><span class="p">,</span> <span class="nb">dict</span><span class="p">)</span> </pre></div> </div> <p>You can of course also override other class methods (or add new methods); for example defining a custom <a class="reference internal" href="#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__call__()</span></code></a> method in the metaclass allows custom behavior when the class is called, e.g. not always creating a new instance.</p> <dl class="data"> <dt id="__metaclass__"> <code class="descname">__metaclass__</code><a class="headerlink" href="#__metaclass__" title="Permalink to this definition">¶</a></dt> <dd><p>This variable can be any callable accepting arguments for <code class="docutils literal notranslate"><span class="pre">name</span></code>, <code class="docutils literal notranslate"><span class="pre">bases</span></code>, and <code class="docutils literal notranslate"><span class="pre">dict</span></code>. Upon class creation, the callable is used instead of the built-in <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-func docutils literal notranslate"><span class="pre">type()</span></code></a>.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.2.</span></p> </div> </dd></dl> <p>The appropriate metaclass is determined by the following precedence rules:</p> <ul class="simple"> <li>If <code class="docutils literal notranslate"><span class="pre">dict['__metaclass__']</span></code> exists, it is used.</li> <li>Otherwise, if there is at least one base class, its metaclass is used (this looks for a <em>__class__</em> attribute first and if not found, uses its type).</li> <li>Otherwise, if a global variable named __metaclass__ exists, it is used.</li> <li>Otherwise, the old-style, classic metaclass (types.ClassType) is used.</li> </ul> <p>The potential uses for metaclasses are boundless. Some ideas that have been explored including logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization.</p> </div> <div class="section" id="customizing-instance-and-subclass-checks"> <h3>3.4.4. Customizing instance and subclass checks<a class="headerlink" href="#customizing-instance-and-subclass-checks" title="Permalink to this headline">¶</a></h3> <div class="versionadded"> <p><span class="versionmodified">New in version 2.6.</span></p> </div> <p>The following methods are used to override the default behavior of the <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate"><span class="pre">isinstance()</span></code></a> and <a class="reference internal" href="../library/functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">issubclass()</span></code></a> built-in functions.</p> <p>In particular, the metaclass <a class="reference internal" href="../library/abc.html#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal notranslate"><span class="pre">abc.ABCMeta</span></code></a> implements these methods in order to allow the addition of Abstract Base Classes (ABCs) as “virtual base classes” to any class or type (including built-in types), including other ABCs.</p> <dl class="method"> <dt id="class.__instancecheck__"> <code class="descclassname">class.</code><code class="descname">__instancecheck__</code><span class="sig-paren">(</span><em>self</em>, <em>instance</em><span class="sig-paren">)</span><a class="headerlink" href="#class.__instancecheck__" title="Permalink to this definition">¶</a></dt> <dd><p>Return true if <em>instance</em> should be considered a (direct or indirect) instance of <em>class</em>. If defined, called to implement <code class="docutils literal notranslate"><span class="pre">isinstance(instance,</span> <span class="pre">class)</span></code>.</p> </dd></dl> <dl class="method"> <dt id="class.__subclasscheck__"> <code class="descclassname">class.</code><code class="descname">__subclasscheck__</code><span class="sig-paren">(</span><em>self</em>, <em>subclass</em><span class="sig-paren">)</span><a class="headerlink" href="#class.__subclasscheck__" title="Permalink to this definition">¶</a></dt> <dd><p>Return true if <em>subclass</em> should be considered a (direct or indirect) subclass of <em>class</em>. If defined, called to implement <code class="docutils literal notranslate"><span class="pre">issubclass(subclass,</span> <span class="pre">class)</span></code>.</p> </dd></dl> <p>Note that these methods are looked up on the type (metaclass) of a class. They cannot be defined as class methods in the actual class. This is consistent with the lookup of special methods that are called on instances, only in this case the instance is itself a class.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-86"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3119"><strong>PEP 3119</strong></a> - Introducing Abstract Base Classes</dt> <dd>Includes the specification for customizing <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate"><span class="pre">isinstance()</span></code></a> and <a class="reference internal" href="../library/functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">issubclass()</span></code></a> behavior through <a class="reference internal" href="#class.__instancecheck__" title="class.__instancecheck__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__instancecheck__()</span></code></a> and <a class="reference internal" href="#class.__subclasscheck__" title="class.__subclasscheck__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__subclasscheck__()</span></code></a>, with motivation for this functionality in the context of adding Abstract Base Classes (see the <a class="reference internal" href="../library/abc.html#module-abc" title="abc: Abstract base classes according to PEP 3119."><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code></a> module) to the language.</dd> </dl> </div> </div> <div class="section" id="emulating-callable-objects"> <span id="callable-types"></span><h3>3.4.5. Emulating callable objects<a class="headerlink" href="#emulating-callable-objects" title="Permalink to this headline">¶</a></h3> <dl class="method"> <dt id="object.__call__"> <code class="descclassname">object.</code><code class="descname">__call__</code><span class="sig-paren">(</span><em>self</em><span class="optional">[</span>, <em>args...</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__call__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-87">Called when the instance is “called” as a function; if this method is defined, <code class="docutils literal notranslate"><span class="pre">x(arg1,</span> <span class="pre">arg2,</span> <span class="pre">...)</span></code> is a shorthand for <code class="docutils literal notranslate"><span class="pre">x.__call__(arg1,</span> <span class="pre">arg2,</span> <span class="pre">...)</span></code>.</p> </dd></dl> </div> <div class="section" id="emulating-container-types"> <span id="sequence-types"></span><h3>3.4.6. Emulating container types<a class="headerlink" href="#emulating-container-types" title="Permalink to this headline">¶</a></h3> <p>The following methods can be defined to implement container objects. Containers usually are sequences (such as lists or tuples) or mappings (like dictionaries), but can represent other containers as well. The first set of methods is used either to emulate a sequence or to emulate a mapping; the difference is that for a sequence, the allowable keys should be the integers <em>k</em> for which <code class="docutils literal notranslate"><span class="pre">0</span> <span class="pre"><=</span> <span class="pre">k</span> <span class="pre"><</span> <span class="pre">N</span></code> where <em>N</em> is the length of the sequence, or slice objects, which define a range of items. (For backwards compatibility, the method <a class="reference internal" href="#object.__getslice__" title="object.__getslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getslice__()</span></code></a> (see below) can also be defined to handle simple, but not extended slices.) It is also recommended that mappings provide the methods <code class="xref py py-meth docutils literal notranslate"><span class="pre">keys()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">values()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">items()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">has_key()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">clear()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">setdefault()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">iterkeys()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">itervalues()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">iteritems()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">pop()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">popitem()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">copy()</span></code>, and <code class="xref py py-meth docutils literal notranslate"><span class="pre">update()</span></code> behaving similar to those for Python’s standard dictionary objects. The <a class="reference internal" href="../library/userdict.html#module-UserDict" title="UserDict: Class wrapper for dictionary objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">UserDict</span></code></a> module provides a <code class="xref py py-class docutils literal notranslate"><span class="pre">DictMixin</span></code> class to help create those methods from a base set of <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, <a class="reference internal" href="#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setitem__()</span></code></a>, <a class="reference internal" href="#object.__delitem__" title="object.__delitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delitem__()</span></code></a>, and <code class="xref py py-meth docutils literal notranslate"><span class="pre">keys()</span></code>. Mutable sequences should provide methods <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">count()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">index()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">extend()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">insert()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">pop()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">remove()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">reverse()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">sort()</span></code>, like Python standard list objects. Finally, sequence types should implement addition (meaning concatenation) and multiplication (meaning repetition) by defining the methods <a class="reference internal" href="#object.__add__" title="object.__add__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__add__()</span></code></a>, <a class="reference internal" href="#object.__radd__" title="object.__radd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__radd__()</span></code></a>, <a class="reference internal" href="#object.__iadd__" title="object.__iadd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iadd__()</span></code></a>, <a class="reference internal" href="#object.__mul__" title="object.__mul__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__mul__()</span></code></a>, <a class="reference internal" href="#object.__rmul__" title="object.__rmul__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rmul__()</span></code></a> and <a class="reference internal" href="#object.__imul__" title="object.__imul__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__imul__()</span></code></a> described below; they should not define <a class="reference internal" href="#object.__coerce__" title="object.__coerce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__coerce__()</span></code></a> or other numerical operators. It is recommended that both mappings and sequences implement the <a class="reference internal" href="#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__contains__()</span></code></a> method to allow efficient use of the <code class="docutils literal notranslate"><span class="pre">in</span></code> operator; for mappings, <code class="docutils literal notranslate"><span class="pre">in</span></code> should be equivalent of <code class="xref py py-meth docutils literal notranslate"><span class="pre">has_key()</span></code>; for sequences, it should search through the values. It is further recommended that both mappings and sequences implement the <a class="reference internal" href="#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> method to allow efficient iteration through the container; for mappings, <a class="reference internal" href="#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> should be the same as <code class="xref py py-meth docutils literal notranslate"><span class="pre">iterkeys()</span></code>; for sequences, it should iterate through the values.</p> <dl class="method"> <dt id="object.__len__"> <code class="descclassname">object.</code><code class="descname">__len__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__len__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-88">Called to implement the built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a>. Should return the length of the object, an integer <code class="docutils literal notranslate"><span class="pre">>=</span></code> 0. Also, an object that doesn’t define a <a class="reference internal" href="#object.__nonzero__" title="object.__nonzero__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__nonzero__()</span></code></a> method and whose <a class="reference internal" href="#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> method returns zero is considered to be false in a Boolean context.</p> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> In CPython, the length is required to be at most <a class="reference internal" href="../library/sys.html#sys.maxsize" title="sys.maxsize"><code class="xref py py-attr docutils literal notranslate"><span class="pre">sys.maxsize</span></code></a>. If the length is larger than <code class="xref py py-attr docutils literal notranslate"><span class="pre">sys.maxsize</span></code> some features (such as <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a>) may raise <a class="reference internal" href="../library/exceptions.html#exceptions.OverflowError" title="exceptions.OverflowError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OverflowError</span></code></a>. To prevent raising <code class="xref py py-exc docutils literal notranslate"><span class="pre">OverflowError</span></code> by truth value testing, an object must define a <a class="reference internal" href="#object.__nonzero__" title="object.__nonzero__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__nonzero__()</span></code></a> method.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__getitem__"> <code class="descclassname">object.</code><code class="descname">__getitem__</code><span class="sig-paren">(</span><em>self</em>, <em>key</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__getitem__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-89">Called to implement evaluation of <code class="docutils literal notranslate"><span class="pre">self[key]</span></code>. For sequence types, the accepted keys should be integers and slice objects. Note that the special interpretation of negative indexes (if the class wishes to emulate a sequence type) is up to the <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method. If <em>key</em> is of an inappropriate type, <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> may be raised; if of a value outside the set of indexes for the sequence (after any special interpretation of negative values), <a class="reference internal" href="../library/exceptions.html#exceptions.IndexError" title="exceptions.IndexError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IndexError</span></code></a> should be raised. For mapping types, if <em>key</em> is missing (not in the container), <a class="reference internal" href="../library/exceptions.html#exceptions.KeyError" title="exceptions.KeyError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyError</span></code></a> should be raised.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last"><a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> loops expect that an <a class="reference internal" href="../library/exceptions.html#exceptions.IndexError" title="exceptions.IndexError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IndexError</span></code></a> will be raised for illegal indexes to allow proper detection of the end of the sequence.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__setitem__"> <code class="descclassname">object.</code><code class="descname">__setitem__</code><span class="sig-paren">(</span><em>self</em>, <em>key</em>, <em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__setitem__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement assignment to <code class="docutils literal notranslate"><span class="pre">self[key]</span></code>. Same note as for <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>. This should only be implemented for mappings if the objects support changes to the values for keys, or if new keys can be added, or for sequences if elements can be replaced. The same exceptions should be raised for improper <em>key</em> values as for the <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method.</p> </dd></dl> <dl class="method"> <dt id="object.__delitem__"> <code class="descclassname">object.</code><code class="descname">__delitem__</code><span class="sig-paren">(</span><em>self</em>, <em>key</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__delitem__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement deletion of <code class="docutils literal notranslate"><span class="pre">self[key]</span></code>. Same note as for <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>. This should only be implemented for mappings if the objects support removal of keys, or for sequences if elements can be removed from the sequence. The same exceptions should be raised for improper <em>key</em> values as for the <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method.</p> </dd></dl> <dl class="method"> <dt id="object.__missing__"> <code class="descclassname">object.</code><code class="descname">__missing__</code><span class="sig-paren">(</span><em>self</em>, <em>key</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__missing__" title="Permalink to this definition">¶</a></dt> <dd><p>Called by <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a>.<a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> to implement <code class="docutils literal notranslate"><span class="pre">self[key]</span></code> for dict subclasses when key is not in the dictionary.</p> </dd></dl> <dl class="method"> <dt id="object.__iter__"> <code class="descclassname">object.</code><code class="descname">__iter__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__iter__" title="Permalink to this definition">¶</a></dt> <dd><p>This method is called when an iterator is required for a container. This method should return a new iterator object that can iterate over all the objects in the container. For mappings, it should iterate over the keys of the container, and should also be made available as the method <code class="xref py py-meth docutils literal notranslate"><span class="pre">iterkeys()</span></code>.</p> <p>Iterator objects also need to implement this method; they are required to return themselves. For more information on iterator objects, see <a class="reference internal" href="../library/stdtypes.html#typeiter"><span class="std std-ref">Iterator Types</span></a>.</p> </dd></dl> <dl class="method"> <dt id="object.__reversed__"> <code class="descclassname">object.</code><code class="descname">__reversed__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__reversed__" title="Permalink to this definition">¶</a></dt> <dd><p>Called (if present) by the <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate"><span class="pre">reversed()</span></code></a> built-in to implement reverse iteration. It should return a new iterator object that iterates over all the objects in the container in reverse order.</p> <p>If the <a class="reference internal" href="#object.__reversed__" title="object.__reversed__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reversed__()</span></code></a> method is not provided, the <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate"><span class="pre">reversed()</span></code></a> built-in will fall back to using the sequence protocol (<a class="reference internal" href="#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> and <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>). Objects that support the sequence protocol should only provide <a class="reference internal" href="#object.__reversed__" title="object.__reversed__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reversed__()</span></code></a> if they can provide an implementation that is more efficient than the one provided by <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate"><span class="pre">reversed()</span></code></a>.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.6.</span></p> </div> </dd></dl> <p>The membership test operators (<a class="reference internal" href="expressions.html#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> and <a class="reference internal" href="expressions.html#not-in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code></a>) are normally implemented as an iteration through a sequence. However, container objects can supply the following special method with a more efficient implementation, which also does not require the object be a sequence.</p> <dl class="method"> <dt id="object.__contains__"> <code class="descclassname">object.</code><code class="descname">__contains__</code><span class="sig-paren">(</span><em>self</em>, <em>item</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__contains__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement membership test operators. Should return true if <em>item</em> is in <em>self</em>, false otherwise. For mapping objects, this should consider the keys of the mapping rather than the values or the key-item pairs.</p> <p>For objects that don’t define <a class="reference internal" href="#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__contains__()</span></code></a>, the membership test first tries iteration via <a class="reference internal" href="#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a>, then the old sequence iteration protocol via <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, see <a class="reference internal" href="expressions.html#membership-test-details"><span class="std std-ref">this section in the language reference</span></a>.</p> </dd></dl> </div> <div class="section" id="additional-methods-for-emulation-of-sequence-types"> <span id="sequence-methods"></span><h3>3.4.7. Additional methods for emulation of sequence types<a class="headerlink" href="#additional-methods-for-emulation-of-sequence-types" title="Permalink to this headline">¶</a></h3> <p>The following optional methods can be defined to further emulate sequence objects. Immutable sequences methods should at most only define <a class="reference internal" href="#object.__getslice__" title="object.__getslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getslice__()</span></code></a>; mutable sequences might define all three methods.</p> <dl class="method"> <dt id="object.__getslice__"> <code class="descclassname">object.</code><code class="descname">__getslice__</code><span class="sig-paren">(</span><em>self</em>, <em>i</em>, <em>j</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__getslice__" title="Permalink to this definition">¶</a></dt> <dd><div class="deprecated"> <p><span class="versionmodified">Deprecated since version 2.0: </span>Support slice objects as parameters to the <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method. (However, built-in types in CPython currently still implement <a class="reference internal" href="#object.__getslice__" title="object.__getslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getslice__()</span></code></a>. Therefore, you have to override it in derived classes when implementing slicing.)</p> </div> <p>Called to implement evaluation of <code class="docutils literal notranslate"><span class="pre">self[i:j]</span></code>. The returned object should be of the same type as <em>self</em>. Note that missing <em>i</em> or <em>j</em> in the slice expression are replaced by zero or <a class="reference internal" href="../library/sys.html#sys.maxsize" title="sys.maxsize"><code class="xref py py-attr docutils literal notranslate"><span class="pre">sys.maxsize</span></code></a>, respectively. If negative indexes are used in the slice, the length of the sequence is added to that index. If the instance does not implement the <a class="reference internal" href="#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__len__()</span></code></a> method, an <a class="reference internal" href="../library/exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> is raised. No guarantee is made that indexes adjusted this way are not still negative. Indexes which are greater than the length of the sequence are not modified. If no <a class="reference internal" href="#object.__getslice__" title="object.__getslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getslice__()</span></code></a> is found, a slice object is created instead, and passed to <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> instead.</p> </dd></dl> <dl class="method"> <dt id="object.__setslice__"> <code class="descclassname">object.</code><code class="descname">__setslice__</code><span class="sig-paren">(</span><em>self</em>, <em>i</em>, <em>j</em>, <em>sequence</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__setslice__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement assignment to <code class="docutils literal notranslate"><span class="pre">self[i:j]</span></code>. Same notes for <em>i</em> and <em>j</em> as for <a class="reference internal" href="#object.__getslice__" title="object.__getslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getslice__()</span></code></a>.</p> <p>This method is deprecated. If no <a class="reference internal" href="#object.__setslice__" title="object.__setslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setslice__()</span></code></a> is found, or for extended slicing of the form <code class="docutils literal notranslate"><span class="pre">self[i:j:k]</span></code>, a slice object is created, and passed to <a class="reference internal" href="#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setitem__()</span></code></a>, instead of <a class="reference internal" href="#object.__setslice__" title="object.__setslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setslice__()</span></code></a> being called.</p> </dd></dl> <dl class="method"> <dt id="object.__delslice__"> <code class="descclassname">object.</code><code class="descname">__delslice__</code><span class="sig-paren">(</span><em>self</em>, <em>i</em>, <em>j</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__delslice__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement deletion of <code class="docutils literal notranslate"><span class="pre">self[i:j]</span></code>. Same notes for <em>i</em> and <em>j</em> as for <a class="reference internal" href="#object.__getslice__" title="object.__getslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getslice__()</span></code></a>. This method is deprecated. If no <a class="reference internal" href="#object.__delslice__" title="object.__delslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delslice__()</span></code></a> is found, or for extended slicing of the form <code class="docutils literal notranslate"><span class="pre">self[i:j:k]</span></code>, a slice object is created, and passed to <a class="reference internal" href="#object.__delitem__" title="object.__delitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delitem__()</span></code></a>, instead of <a class="reference internal" href="#object.__delslice__" title="object.__delslice__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delslice__()</span></code></a> being called.</p> </dd></dl> <p>Notice that these methods are only invoked when a single slice with a single colon is used, and the slice method is available. For slice operations involving extended slice notation, or in absence of the slice methods, <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, <a class="reference internal" href="#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setitem__()</span></code></a> or <a class="reference internal" href="#object.__delitem__" title="object.__delitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delitem__()</span></code></a> is called with a slice object as argument.</p> <p>The following example demonstrate how to make your program or module compatible with earlier versions of Python (assuming that methods <a class="reference internal" href="#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, <a class="reference internal" href="#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setitem__()</span></code></a> and <a class="reference internal" href="#object.__delitem__" title="object.__delitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delitem__()</span></code></a> support slice objects as arguments):</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MyClass</span><span class="p">:</span> <span class="o">...</span> <span class="k">def</span> <span class="nf">__getitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">index</span><span class="p">):</span> <span class="o">...</span> <span class="k">def</span> <span class="nf">__setitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">index</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span> <span class="o">...</span> <span class="k">def</span> <span class="nf">__delitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">index</span><span class="p">):</span> <span class="o">...</span> <span class="k">if</span> <span class="n">sys</span><span class="o">.</span><span class="n">version_info</span> <span class="o"><</span> <span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">0</span><span class="p">):</span> <span class="c1"># They won't be defined if version is at least 2.0 final</span> <span class="k">def</span> <span class="nf">__getslice__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span><span class="p">):</span> <span class="k">return</span> <span class="bp">self</span><span class="p">[</span><span class="nb">max</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">i</span><span class="p">):</span><span class="nb">max</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">j</span><span class="p">):]</span> <span class="k">def</span> <span class="nf">__setslice__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span><span class="p">,</span> <span class="n">seq</span><span class="p">):</span> <span class="bp">self</span><span class="p">[</span><span class="nb">max</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">i</span><span class="p">):</span><span class="nb">max</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">j</span><span class="p">):]</span> <span class="o">=</span> <span class="n">seq</span> <span class="k">def</span> <span class="nf">__delslice__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span><span class="p">):</span> <span class="k">del</span> <span class="bp">self</span><span class="p">[</span><span class="nb">max</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">i</span><span class="p">):</span><span class="nb">max</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">j</span><span class="p">):]</span> <span class="o">...</span> </pre></div> </div> <p>Note the calls to <a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a>; these are necessary because of the handling of negative indices before the <code class="xref py py-meth docutils literal notranslate"><span class="pre">__*slice__()</span></code> methods are called. When negative indexes are used, the <code class="xref py py-meth docutils literal notranslate"><span class="pre">__*item__()</span></code> methods receive them as provided, but the <code class="xref py py-meth docutils literal notranslate"><span class="pre">__*slice__()</span></code> methods get a “cooked” form of the index values. For each negative index value, the length of the sequence is added to the index before calling the method (which may still result in a negative index); this is the customary handling of negative indexes by the built-in sequence types, and the <code class="xref py py-meth docutils literal notranslate"><span class="pre">__*item__()</span></code> methods are expected to do this as well. However, since they should already be doing that, negative indexes cannot be passed in; they must be constrained to the bounds of the sequence before being passed to the <code class="xref py py-meth docutils literal notranslate"><span class="pre">__*item__()</span></code> methods. Calling <code class="docutils literal notranslate"><span class="pre">max(0,</span> <span class="pre">i)</span></code> conveniently returns the proper value.</p> </div> <div class="section" id="emulating-numeric-types"> <span id="numeric-types"></span><h3>3.4.8. Emulating numeric types<a class="headerlink" href="#emulating-numeric-types" title="Permalink to this headline">¶</a></h3> <p>The following methods can be defined to emulate numeric objects. Methods corresponding to operations that are not supported by the particular kind of number implemented (e.g., bitwise operations for non-integral numbers) should be left undefined.</p> <dl class="method"> <dt id="object.__add__"> <code class="descclassname">object.</code><code class="descname">__add__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__add__" title="Permalink to this definition">¶</a></dt> <dt id="object.__sub__"> <code class="descclassname">object.</code><code class="descname">__sub__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__sub__" title="Permalink to this definition">¶</a></dt> <dt id="object.__mul__"> <code class="descclassname">object.</code><code class="descname">__mul__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__mul__" title="Permalink to this definition">¶</a></dt> <dt id="object.__floordiv__"> <code class="descclassname">object.</code><code class="descname">__floordiv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__floordiv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__mod__"> <code class="descclassname">object.</code><code class="descname">__mod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__mod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__divmod__"> <code class="descclassname">object.</code><code class="descname">__divmod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__divmod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__pow__"> <code class="descclassname">object.</code><code class="descname">__pow__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="optional">[</span>, <em>modulo</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__pow__" title="Permalink to this definition">¶</a></dt> <dt id="object.__lshift__"> <code class="descclassname">object.</code><code class="descname">__lshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__lshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rshift__"> <code class="descclassname">object.</code><code class="descname">__rshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__and__"> <code class="descclassname">object.</code><code class="descname">__and__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__and__" title="Permalink to this definition">¶</a></dt> <dt id="object.__xor__"> <code class="descclassname">object.</code><code class="descname">__xor__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__xor__" title="Permalink to this definition">¶</a></dt> <dt id="object.__or__"> <code class="descclassname">object.</code><code class="descname">__or__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__or__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-90">These methods are called to implement the binary arithmetic operations (<code class="docutils literal notranslate"><span class="pre">+</span></code>, <code class="docutils literal notranslate"><span class="pre">-</span></code>, <code class="docutils literal notranslate"><span class="pre">*</span></code>, <code class="docutils literal notranslate"><span class="pre">//</span></code>, <code class="docutils literal notranslate"><span class="pre">%</span></code>, <a class="reference internal" href="../library/functions.html#divmod" title="divmod"><code class="xref py py-func docutils literal notranslate"><span class="pre">divmod()</span></code></a>, <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate"><span class="pre">pow()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">**</span></code>, <code class="docutils literal notranslate"><span class="pre"><<</span></code>, <code class="docutils literal notranslate"><span class="pre">>></span></code>, <code class="docutils literal notranslate"><span class="pre">&</span></code>, <code class="docutils literal notranslate"><span class="pre">^</span></code>, <code class="docutils literal notranslate"><span class="pre">|</span></code>). For instance, to evaluate the expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">+</span> <span class="pre">y</span></code>, where <em>x</em> is an instance of a class that has an <a class="reference internal" href="#object.__add__" title="object.__add__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__add__()</span></code></a> method, <code class="docutils literal notranslate"><span class="pre">x.__add__(y)</span></code> is called. The <a class="reference internal" href="#object.__divmod__" title="object.__divmod__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__divmod__()</span></code></a> method should be the equivalent to using <a class="reference internal" href="#object.__floordiv__" title="object.__floordiv__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__floordiv__()</span></code></a> and <a class="reference internal" href="#object.__mod__" title="object.__mod__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__mod__()</span></code></a>; it should not be related to <a class="reference internal" href="#object.__truediv__" title="object.__truediv__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__truediv__()</span></code></a> (described below). Note that <a class="reference internal" href="#object.__pow__" title="object.__pow__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__pow__()</span></code></a> should be defined to accept an optional third argument if the ternary version of the built-in <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate"><span class="pre">pow()</span></code></a> function is to be supported.</p> <p>If one of those methods does not support the operation with the supplied arguments, it should return <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>.</p> </dd></dl> <dl class="method"> <dt id="object.__div__"> <code class="descclassname">object.</code><code class="descname">__div__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__div__" title="Permalink to this definition">¶</a></dt> <dt id="object.__truediv__"> <code class="descclassname">object.</code><code class="descname">__truediv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__truediv__" title="Permalink to this definition">¶</a></dt> <dd><p>The division operator (<code class="docutils literal notranslate"><span class="pre">/</span></code>) is implemented by these methods. The <a class="reference internal" href="#object.__truediv__" title="object.__truediv__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__truediv__()</span></code></a> method is used when <code class="docutils literal notranslate"><span class="pre">__future__.division</span></code> is in effect, otherwise <a class="reference internal" href="#object.__div__" title="object.__div__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__div__()</span></code></a> is used. If only one of these two methods is defined, the object will not support division in the alternate context; <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> will be raised instead.</p> </dd></dl> <dl class="method"> <dt id="object.__radd__"> <code class="descclassname">object.</code><code class="descname">__radd__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__radd__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rsub__"> <code class="descclassname">object.</code><code class="descname">__rsub__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rsub__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rmul__"> <code class="descclassname">object.</code><code class="descname">__rmul__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rmul__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rdiv__"> <code class="descclassname">object.</code><code class="descname">__rdiv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rdiv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rtruediv__"> <code class="descclassname">object.</code><code class="descname">__rtruediv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rtruediv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rfloordiv__"> <code class="descclassname">object.</code><code class="descname">__rfloordiv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rfloordiv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rmod__"> <code class="descclassname">object.</code><code class="descname">__rmod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rmod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rdivmod__"> <code class="descclassname">object.</code><code class="descname">__rdivmod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rdivmod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rpow__"> <code class="descclassname">object.</code><code class="descname">__rpow__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rpow__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rlshift__"> <code class="descclassname">object.</code><code class="descname">__rlshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rlshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rrshift__"> <code class="descclassname">object.</code><code class="descname">__rrshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rrshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rand__"> <code class="descclassname">object.</code><code class="descname">__rand__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rand__" title="Permalink to this definition">¶</a></dt> <dt id="object.__rxor__"> <code class="descclassname">object.</code><code class="descname">__rxor__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__rxor__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ror__"> <code class="descclassname">object.</code><code class="descname">__ror__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ror__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-91">These methods are called to implement the binary arithmetic operations (<code class="docutils literal notranslate"><span class="pre">+</span></code>, <code class="docutils literal notranslate"><span class="pre">-</span></code>, <code class="docutils literal notranslate"><span class="pre">*</span></code>, <code class="docutils literal notranslate"><span class="pre">/</span></code>, <code class="docutils literal notranslate"><span class="pre">%</span></code>, <a class="reference internal" href="../library/functions.html#divmod" title="divmod"><code class="xref py py-func docutils literal notranslate"><span class="pre">divmod()</span></code></a>, <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate"><span class="pre">pow()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">**</span></code>, <code class="docutils literal notranslate"><span class="pre"><<</span></code>, <code class="docutils literal notranslate"><span class="pre">>></span></code>, <code class="docutils literal notranslate"><span class="pre">&</span></code>, <code class="docutils literal notranslate"><span class="pre">^</span></code>, <code class="docutils literal notranslate"><span class="pre">|</span></code>) with reflected (swapped) operands. These functions are only called if the left operand does not support the corresponding operation and the operands are of different types. <a class="footnote-reference" href="#id6" id="id3">[2]</a> For instance, to evaluate the expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">-</span> <span class="pre">y</span></code>, where <em>y</em> is an instance of a class that has an <a class="reference internal" href="#object.__rsub__" title="object.__rsub__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rsub__()</span></code></a> method, <code class="docutils literal notranslate"><span class="pre">y.__rsub__(x)</span></code> is called if <code class="docutils literal notranslate"><span class="pre">x.__sub__(y)</span></code> returns <em>NotImplemented</em>.</p> <p id="index-92">Note that ternary <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate"><span class="pre">pow()</span></code></a> will not try calling <a class="reference internal" href="#object.__rpow__" title="object.__rpow__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rpow__()</span></code></a> (the coercion rules would become too complicated).</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">If the right operand’s type is a subclass of the left operand’s type and that subclass provides the reflected method for the operation, this method will be called before the left operand’s non-reflected method. This behavior allows subclasses to override their ancestors’ operations.</p> </div> </dd></dl> <dl class="method"> <dt id="object.__iadd__"> <code class="descclassname">object.</code><code class="descname">__iadd__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__iadd__" title="Permalink to this definition">¶</a></dt> <dt id="object.__isub__"> <code class="descclassname">object.</code><code class="descname">__isub__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__isub__" title="Permalink to this definition">¶</a></dt> <dt id="object.__imul__"> <code class="descclassname">object.</code><code class="descname">__imul__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__imul__" title="Permalink to this definition">¶</a></dt> <dt id="object.__idiv__"> <code class="descclassname">object.</code><code class="descname">__idiv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__idiv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__itruediv__"> <code class="descclassname">object.</code><code class="descname">__itruediv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__itruediv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ifloordiv__"> <code class="descclassname">object.</code><code class="descname">__ifloordiv__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ifloordiv__" title="Permalink to this definition">¶</a></dt> <dt id="object.__imod__"> <code class="descclassname">object.</code><code class="descname">__imod__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__imod__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ipow__"> <code class="descclassname">object.</code><code class="descname">__ipow__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="optional">[</span>, <em>modulo</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__ipow__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ilshift__"> <code class="descclassname">object.</code><code class="descname">__ilshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ilshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__irshift__"> <code class="descclassname">object.</code><code class="descname">__irshift__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__irshift__" title="Permalink to this definition">¶</a></dt> <dt id="object.__iand__"> <code class="descclassname">object.</code><code class="descname">__iand__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__iand__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ixor__"> <code class="descclassname">object.</code><code class="descname">__ixor__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ixor__" title="Permalink to this definition">¶</a></dt> <dt id="object.__ior__"> <code class="descclassname">object.</code><code class="descname">__ior__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__ior__" title="Permalink to this definition">¶</a></dt> <dd><p>These methods are called to implement the augmented arithmetic assignments (<code class="docutils literal notranslate"><span class="pre">+=</span></code>, <code class="docutils literal notranslate"><span class="pre">-=</span></code>, <code class="docutils literal notranslate"><span class="pre">*=</span></code>, <code class="docutils literal notranslate"><span class="pre">/=</span></code>, <code class="docutils literal notranslate"><span class="pre">//=</span></code>, <code class="docutils literal notranslate"><span class="pre">%=</span></code>, <code class="docutils literal notranslate"><span class="pre">**=</span></code>, <code class="docutils literal notranslate"><span class="pre"><<=</span></code>, <code class="docutils literal notranslate"><span class="pre">>>=</span></code>, <code class="docutils literal notranslate"><span class="pre">&=</span></code>, <code class="docutils literal notranslate"><span class="pre">^=</span></code>, <code class="docutils literal notranslate"><span class="pre">|=</span></code>). These methods should attempt to do the operation in-place (modifying <em>self</em>) and return the result (which could be, but does not have to be, <em>self</em>). If a specific method is not defined, the augmented assignment falls back to the normal methods. For instance, to execute the statement <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">+=</span> <span class="pre">y</span></code>, where <em>x</em> is an instance of a class that has an <a class="reference internal" href="#object.__iadd__" title="object.__iadd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iadd__()</span></code></a> method, <code class="docutils literal notranslate"><span class="pre">x.__iadd__(y)</span></code> is called. If <em>x</em> is an instance of a class that does not define a <a class="reference internal" href="#object.__iadd__" title="object.__iadd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iadd__()</span></code></a> method, <code class="docutils literal notranslate"><span class="pre">x.__add__(y)</span></code> and <code class="docutils literal notranslate"><span class="pre">y.__radd__(x)</span></code> are considered, as with the evaluation of <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">+</span> <span class="pre">y</span></code>.</p> </dd></dl> <dl class="method"> <dt id="object.__neg__"> <code class="descclassname">object.</code><code class="descname">__neg__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__neg__" title="Permalink to this definition">¶</a></dt> <dt id="object.__pos__"> <code class="descclassname">object.</code><code class="descname">__pos__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__pos__" title="Permalink to this definition">¶</a></dt> <dt id="object.__abs__"> <code class="descclassname">object.</code><code class="descname">__abs__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__abs__" title="Permalink to this definition">¶</a></dt> <dt id="object.__invert__"> <code class="descclassname">object.</code><code class="descname">__invert__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__invert__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-93">Called to implement the unary arithmetic operations (<code class="docutils literal notranslate"><span class="pre">-</span></code>, <code class="docutils literal notranslate"><span class="pre">+</span></code>, <a class="reference internal" href="../library/functions.html#abs" title="abs"><code class="xref py py-func docutils literal notranslate"><span class="pre">abs()</span></code></a> and <code class="docutils literal notranslate"><span class="pre">~</span></code>).</p> </dd></dl> <dl class="method"> <dt id="object.__complex__"> <code class="descclassname">object.</code><code class="descname">__complex__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__complex__" title="Permalink to this definition">¶</a></dt> <dt id="object.__int__"> <code class="descclassname">object.</code><code class="descname">__int__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__int__" title="Permalink to this definition">¶</a></dt> <dt id="object.__long__"> <code class="descclassname">object.</code><code class="descname">__long__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__long__" title="Permalink to this definition">¶</a></dt> <dt id="object.__float__"> <code class="descclassname">object.</code><code class="descname">__float__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__float__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-94">Called to implement the built-in functions <a class="reference internal" href="../library/functions.html#complex" title="complex"><code class="xref py py-func docutils literal notranslate"><span class="pre">complex()</span></code></a>, <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate"><span class="pre">int()</span></code></a>, <a class="reference internal" href="../library/functions.html#long" title="long"><code class="xref py py-func docutils literal notranslate"><span class="pre">long()</span></code></a>, and <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-func docutils literal notranslate"><span class="pre">float()</span></code></a>. Should return a value of the appropriate type.</p> </dd></dl> <dl class="method"> <dt id="object.__oct__"> <code class="descclassname">object.</code><code class="descname">__oct__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__oct__" title="Permalink to this definition">¶</a></dt> <dt id="object.__hex__"> <code class="descclassname">object.</code><code class="descname">__hex__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__hex__" title="Permalink to this definition">¶</a></dt> <dd><p id="index-95">Called to implement the built-in functions <a class="reference internal" href="../library/functions.html#oct" title="oct"><code class="xref py py-func docutils literal notranslate"><span class="pre">oct()</span></code></a> and <a class="reference internal" href="../library/functions.html#hex" title="hex"><code class="xref py py-func docutils literal notranslate"><span class="pre">hex()</span></code></a>. Should return a string value.</p> </dd></dl> <dl class="method"> <dt id="object.__index__"> <code class="descclassname">object.</code><code class="descname">__index__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__index__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement <a class="reference internal" href="../library/operator.html#operator.index" title="operator.index"><code class="xref py py-func docutils literal notranslate"><span class="pre">operator.index()</span></code></a>. Also called whenever Python needs an integer object (such as in slicing). Must return an integer (int or long).</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.5.</span></p> </div> </dd></dl> <dl class="method"> <dt id="object.__coerce__"> <code class="descclassname">object.</code><code class="descname">__coerce__</code><span class="sig-paren">(</span><em>self</em>, <em>other</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__coerce__" title="Permalink to this definition">¶</a></dt> <dd><p>Called to implement “mixed-mode” numeric arithmetic. Should either return a 2-tuple containing <em>self</em> and <em>other</em> converted to a common numeric type, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if conversion is impossible. When the common type would be the type of <code class="docutils literal notranslate"><span class="pre">other</span></code>, it is sufficient to return <code class="docutils literal notranslate"><span class="pre">None</span></code>, since the interpreter will also ask the other object to attempt a coercion (but sometimes, if the implementation of the other type cannot be changed, it is useful to do the conversion to the other type here). A return value of <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> is equivalent to returning <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p> </dd></dl> </div> <div class="section" id="coercion-rules"> <span id="id4"></span><h3>3.4.9. Coercion rules<a class="headerlink" href="#coercion-rules" title="Permalink to this headline">¶</a></h3> <p>This section used to document the rules for coercion. As the language has evolved, the coercion rules have become hard to document precisely; documenting what one version of one particular implementation does is undesirable. Instead, here are some informal guidelines regarding coercion. In Python 3, coercion will not be supported.</p> <ul> <li><p class="first">If the left operand of a % operator is a string or Unicode object, no coercion takes place and the string formatting operation is invoked instead.</p> </li> <li><p class="first">It is no longer recommended to define a coercion operation. Mixed-mode operations on types that don’t define coercion pass the original arguments to the operation.</p> </li> <li><p class="first">New-style classes (those derived from <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a>) never invoke the <a class="reference internal" href="#object.__coerce__" title="object.__coerce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__coerce__()</span></code></a> method in response to a binary operator; the only time <a class="reference internal" href="#object.__coerce__" title="object.__coerce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__coerce__()</span></code></a> is invoked is when the built-in function <a class="reference internal" href="../library/functions.html#coerce" title="coerce"><code class="xref py py-func docutils literal notranslate"><span class="pre">coerce()</span></code></a> is called.</p> </li> <li><p class="first">For most intents and purposes, an operator that returns <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> is treated the same as one that is not implemented at all.</p> </li> <li><p class="first">Below, <code class="xref py py-meth docutils literal notranslate"><span class="pre">__op__()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">__rop__()</span></code> are used to signify the generic method names corresponding to an operator; <code class="xref py py-meth docutils literal notranslate"><span class="pre">__iop__()</span></code> is used for the corresponding in-place operator. For example, for the operator ‘<code class="docutils literal notranslate"><span class="pre">+</span></code>’, <a class="reference internal" href="#object.__add__" title="object.__add__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__add__()</span></code></a> and <a class="reference internal" href="#object.__radd__" title="object.__radd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__radd__()</span></code></a> are used for the left and right variant of the binary operator, and <a class="reference internal" href="#object.__iadd__" title="object.__iadd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iadd__()</span></code></a> for the in-place variant.</p> </li> <li><p class="first">For objects <em>x</em> and <em>y</em>, first <code class="docutils literal notranslate"><span class="pre">x.__op__(y)</span></code> is tried. If this is not implemented or returns <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>, <code class="docutils literal notranslate"><span class="pre">y.__rop__(x)</span></code> is tried. If this is also not implemented or returns <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>, a <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is raised. But see the following exception:</p> </li> <li><p class="first">Exception to the previous item: if the left operand is an instance of a built-in type or a new-style class, and the right operand is an instance of a proper subclass of that type or class and overrides the base’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">__rop__()</span></code> method, the right operand’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">__rop__()</span></code> method is tried <em>before</em> the left operand’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">__op__()</span></code> method.</p> <p>This is done so that a subclass can completely override binary operators. Otherwise, the left operand’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">__op__()</span></code> method would always accept the right operand: when an instance of a given class is expected, an instance of a subclass of that class is always acceptable.</p> </li> <li><p class="first">When either operand type defines a coercion, this coercion is called before that type’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">__op__()</span></code> or <code class="xref py py-meth docutils literal notranslate"><span class="pre">__rop__()</span></code> method is called, but no sooner. If the coercion returns an object of a different type for the operand whose coercion is invoked, part of the process is redone using the new object.</p> </li> <li><p class="first">When an in-place operator (like ‘<code class="docutils literal notranslate"><span class="pre">+=</span></code>’) is used, if the left operand implements <code class="xref py py-meth docutils literal notranslate"><span class="pre">__iop__()</span></code>, it is invoked without any coercion. When the operation falls back to <code class="xref py py-meth docutils literal notranslate"><span class="pre">__op__()</span></code> and/or <code class="xref py py-meth docutils literal notranslate"><span class="pre">__rop__()</span></code>, the normal coercion rules apply.</p> </li> <li><p class="first">In <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">+</span> <span class="pre">y</span></code>, if <em>x</em> is a sequence that implements sequence concatenation, sequence concatenation is invoked.</p> </li> <li><p class="first">In <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">*</span> <span class="pre">y</span></code>, if one operand is a sequence that implements sequence repetition, and the other is an integer (<a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a> or <a class="reference internal" href="../library/functions.html#long" title="long"><code class="xref py py-class docutils literal notranslate"><span class="pre">long</span></code></a>), sequence repetition is invoked.</p> </li> <li><p class="first">Rich comparisons (implemented by methods <a class="reference internal" href="#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> and so on) never use coercion. Three-way comparison (implemented by <a class="reference internal" href="#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a>) does use coercion under the same conditions as other binary operations use it.</p> </li> <li><p class="first">In the current implementation, the built-in numeric types <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>, <a class="reference internal" href="../library/functions.html#long" title="long"><code class="xref py py-class docutils literal notranslate"><span class="pre">long</span></code></a>, <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a>, and <a class="reference internal" href="../library/functions.html#complex" title="complex"><code class="xref py py-class docutils literal notranslate"><span class="pre">complex</span></code></a> do not use coercion. All these types implement a <a class="reference internal" href="#object.__coerce__" title="object.__coerce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__coerce__()</span></code></a> method, for use by the built-in <a class="reference internal" href="../library/functions.html#coerce" title="coerce"><code class="xref py py-func docutils literal notranslate"><span class="pre">coerce()</span></code></a> function.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.7: </span>The complex type no longer makes implicit calls to the <a class="reference internal" href="#object.__coerce__" title="object.__coerce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__coerce__()</span></code></a> method for mixed-type binary arithmetic operations.</p> </div> </li> </ul> </div> <div class="section" id="with-statement-context-managers"> <span id="context-managers"></span><h3>3.4.10. With Statement Context Managers<a class="headerlink" href="#with-statement-context-managers" title="Permalink to this headline">¶</a></h3> <div class="versionadded"> <p><span class="versionmodified">New in version 2.5.</span></p> </div> <p>A <em class="dfn">context manager</em> is an object that defines the runtime context to be established when executing a <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement. The context manager handles the entry into, and the exit from, the desired runtime context for the execution of the block of code. Context managers are normally invoked using the <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement (described in section <a class="reference internal" href="compound_stmts.html#with"><span class="std std-ref">The with statement</span></a>), but can also be used by directly invoking their methods.</p> <p id="index-96">Typical uses of context managers include saving and restoring various kinds of global state, locking and unlocking resources, closing opened files, etc.</p> <p>For more information on context managers, see <a class="reference internal" href="../library/stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a>.</p> <dl class="method"> <dt id="object.__enter__"> <code class="descclassname">object.</code><code class="descname">__enter__</code><span class="sig-paren">(</span><em>self</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__enter__" title="Permalink to this definition">¶</a></dt> <dd><p>Enter the runtime context related to this object. The <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement will bind this method’s return value to the target(s) specified in the <a class="reference internal" href="compound_stmts.html#as"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code></a> clause of the statement, if any.</p> </dd></dl> <dl class="method"> <dt id="object.__exit__"> <code class="descclassname">object.</code><code class="descname">__exit__</code><span class="sig-paren">(</span><em>self</em>, <em>exc_type</em>, <em>exc_value</em>, <em>traceback</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__exit__" title="Permalink to this definition">¶</a></dt> <dd><p>Exit the runtime context related to this object. The parameters describe the exception that caused the context to be exited. If the context was exited without an exception, all three arguments will be <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>.</p> <p>If an exception is supplied, and the method wishes to suppress the exception (i.e., prevent it from being propagated), it should return a true value. Otherwise, the exception will be processed normally upon exit from this method.</p> <p>Note that <a class="reference internal" href="#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> methods should not reraise the passed-in exception; this is the caller’s responsibility.</p> </dd></dl> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-97"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0343"><strong>PEP 343</strong></a> - The “with” statement</dt> <dd>The specification, background, and examples for the Python <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</dd> </dl> </div> </div> <div class="section" id="special-method-lookup-for-old-style-classes"> <span id="old-style-special-lookup"></span><h3>3.4.11. Special method lookup for old-style classes<a class="headerlink" href="#special-method-lookup-for-old-style-classes" title="Permalink to this headline">¶</a></h3> <p>For old-style classes, special methods are always looked up in exactly the same way as any other method or attribute. This is the case regardless of whether the method is being looked up explicitly as in <code class="docutils literal notranslate"><span class="pre">x.__getitem__(i)</span></code> or implicitly as in <code class="docutils literal notranslate"><span class="pre">x[i]</span></code>.</p> <p>This behaviour means that special methods may exhibit different behaviour for different instances of a single old-style class if the appropriate special attributes are set differently:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="gp">... </span> <span class="k">pass</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">c1</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">c2</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">c1</span><span class="o">.</span><span class="fm">__len__</span> <span class="o">=</span> <span class="k">lambda</span><span class="p">:</span> <span class="mi">5</span> <span class="gp">>>> </span><span class="n">c2</span><span class="o">.</span><span class="fm">__len__</span> <span class="o">=</span> <span class="k">lambda</span><span class="p">:</span> <span class="mi">9</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">c1</span><span class="p">)</span> <span class="go">5</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">c2</span><span class="p">)</span> <span class="go">9</span> </pre></div> </div> </div> <div class="section" id="special-method-lookup-for-new-style-classes"> <span id="new-style-special-lookup"></span><h3>3.4.12. Special method lookup for new-style classes<a class="headerlink" href="#special-method-lookup-for-new-style-classes" title="Permalink to this headline">¶</a></h3> <p>For new-style classes, implicit invocations of special methods are only guaranteed to work correctly if defined on an object’s type, not in the object’s instance dictionary. That behaviour is the reason why the following code raises an exception (unlike the equivalent example with old-style classes):</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span> <span class="gp">... </span> <span class="k">pass</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="fm">__len__</span> <span class="o">=</span> <span class="k">lambda</span><span class="p">:</span> <span class="mi">5</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">c</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">TypeError</span>: <span class="n">object of type 'C' has no len()</span> </pre></div> </div> <p>The rationale behind this behaviour lies with a number of special methods such as <a class="reference internal" href="#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> and <a class="reference internal" href="#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__repr__()</span></code></a> that are implemented by all objects, including type objects. If the implicit lookup of these methods used the conventional lookup process, they would fail when invoked on the type object itself:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">1</span> <span class="o">.</span><span class="fm">__hash__</span><span class="p">()</span> <span class="o">==</span> <span class="nb">hash</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="go">True</span> <span class="gp">>>> </span><span class="nb">int</span><span class="o">.</span><span class="fm">__hash__</span><span class="p">()</span> <span class="o">==</span> <span class="nb">hash</span><span class="p">(</span><span class="nb">int</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">TypeError</span>: <span class="n">descriptor '__hash__' of 'int' object needs an argument</span> </pre></div> </div> <p>Incorrectly attempting to invoke an unbound method of a class in this way is sometimes referred to as ‘metaclass confusion’, and is avoided by bypassing the instance when looking up special methods:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span><span class="o">.</span><span class="fm">__hash__</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="o">==</span> <span class="nb">hash</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="go">True</span> <span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="nb">int</span><span class="p">)</span><span class="o">.</span><span class="fm">__hash__</span><span class="p">(</span><span class="nb">int</span><span class="p">)</span> <span class="o">==</span> <span class="nb">hash</span><span class="p">(</span><span class="nb">int</span><span class="p">)</span> <span class="go">True</span> </pre></div> </div> <p>In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the <a class="reference internal" href="#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> method even of the object’s metaclass:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Meta</span><span class="p">(</span><span class="nb">type</span><span class="p">):</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__getattribute__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span> <span class="s2">"Metaclass getattribute invoked"</span> <span class="gp">... </span> <span class="k">return</span> <span class="nb">type</span><span class="o">.</span><span class="fm">__getattribute__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span> <span class="gp">... </span> <span class="n">__metaclass__</span> <span class="o">=</span> <span class="n">Meta</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__len__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="gp">... </span> <span class="k">return</span> <span class="mi">10</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__getattribute__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span> <span class="s2">"Class getattribute invoked"</span> <span class="gp">... </span> <span class="k">return</span> <span class="nb">object</span><span class="o">.</span><span class="fm">__getattribute__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="fm">__len__</span><span class="p">()</span> <span class="c1"># Explicit lookup via instance</span> <span class="go">Class getattribute invoked</span> <span class="go">10</span> <span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="n">c</span><span class="p">)</span><span class="o">.</span><span class="fm">__len__</span><span class="p">(</span><span class="n">c</span><span class="p">)</span> <span class="c1"># Explicit lookup via type</span> <span class="go">Metaclass getattribute invoked</span> <span class="go">10</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">c</span><span class="p">)</span> <span class="c1"># Implicit lookup</span> <span class="go">10</span> </pre></div> </div> <p>Bypassing the <a class="reference internal" href="#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method <em>must</em> be set on the class object itself in order to be consistently invoked by the interpreter).</p> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id5" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>It <em>is</em> possible in some cases to change an object’s type, under certain controlled conditions. It generally isn’t a good idea though, since it can lead to some very strange behaviour if it is handled incorrectly.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id6" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id3">[2]</a></td><td>For operands of the same type, it is assumed that if the non-reflected method (such as <a class="reference internal" href="#object.__add__" title="object.__add__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__add__()</span></code></a>) fails the operation is not supported, which is why the reflected method is not called.</td></tr> </tbody> </table> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">3. Data model</a><ul> <li><a class="reference internal" href="#objects-values-and-types">3.1. Objects, values and types</a></li> <li><a class="reference internal" href="#the-standard-type-hierarchy">3.2. The standard type hierarchy</a></li> <li><a class="reference internal" href="#new-style-and-classic-classes">3.3. New-style and classic classes</a></li> <li><a class="reference internal" href="#special-method-names">3.4. Special method names</a><ul> <li><a class="reference internal" href="#basic-customization">3.4.1. Basic customization</a></li> <li><a class="reference internal" href="#customizing-attribute-access">3.4.2. Customizing attribute access</a><ul> <li><a class="reference internal" href="#more-attribute-access-for-new-style-classes">3.4.2.1. More attribute access for new-style classes</a></li> <li><a class="reference internal" href="#implementing-descriptors">3.4.2.2. Implementing Descriptors</a></li> <li><a class="reference internal" href="#invoking-descriptors">3.4.2.3. Invoking Descriptors</a></li> <li><a class="reference internal" href="#slots">3.4.2.4. __slots__</a></li> </ul> </li> <li><a class="reference internal" href="#customizing-class-creation">3.4.3. Customizing class creation</a></li> <li><a class="reference internal" href="#customizing-instance-and-subclass-checks">3.4.4. Customizing instance and subclass checks</a></li> <li><a class="reference internal" href="#emulating-callable-objects">3.4.5. Emulating callable objects</a></li> <li><a class="reference internal" href="#emulating-container-types">3.4.6. Emulating container types</a></li> <li><a class="reference internal" href="#additional-methods-for-emulation-of-sequence-types">3.4.7. Additional methods for emulation of sequence types</a></li> <li><a class="reference internal" href="#emulating-numeric-types">3.4.8. Emulating numeric types</a></li> <li><a class="reference internal" href="#coercion-rules">3.4.9. Coercion rules</a></li> <li><a class="reference internal" href="#with-statement-context-managers">3.4.10. With Statement Context Managers</a></li> <li><a class="reference internal" href="#special-method-lookup-for-old-style-classes">3.4.11. Special method lookup for old-style classes</a></li> <li><a class="reference internal" href="#special-method-lookup-for-new-style-classes">3.4.12. Special method lookup for new-style classes</a></li> </ul> </li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="lexical_analysis.html" title="previous chapter">2. Lexical analysis</a></p> <h4>Next topic</h4> <p class="topless"><a href="executionmodel.html" title="next chapter">4. 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