python_doc_md/Intermediate/reference/datamodel.md

Data model {#datamodel}

Objects, values and types {#objects}

::: index single: object single: data :::

Objects{.interpreted-text role="dfn"} are Python's abstraction for data. All data in a Python program is represented by objects or by relations between objects. Even code is represented by objects.

::: index pair: built-in function; id pair: built-in function; type single: identity of an object single: value of an object single: type of an object single: mutable object single: immutable object :::

Every object has an identity, a type and a value. An object's identity never changes once it has been created; you may think of it as the object's address in memory. The is{.interpreted-text role="keyword"} operator compares the identity of two objects; the id{.interpreted-text role="func"} function returns an integer representing its identity.

::: impl-detail For CPython, id(x) is the memory address where x is stored. :::

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 type{.interpreted-text role="func"} function returns an object's type (which is an object itself). Like its identity, an object's type{.interpreted-text role="dfn"} is also unchangeable. [^1]

The value of some objects can change. Objects whose value can change are said to be mutable; objects whose value is unchangeable once they are created are called immutable. (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.

::: index single: garbage collection single: reference counting single: unreachable object :::

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.

::: impl-detail 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 gc{.interpreted-text role="mod"} 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 (so you should always close files explicitly). :::

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 try{.interpreted-text role="keyword"}...except{.interpreted-text role="keyword"} statement may keep objects alive.

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 !close{.interpreted-text role="meth"} method. Programs are strongly recommended to explicitly close such objects. The try{.interpreted-text role="keyword"}...finally{.interpreted-text role="keyword"} statement and the with{.interpreted-text role="keyword"} statement provide convenient ways to do this.

::: index single: container :::

Some objects contain references to other objects; these are called containers. 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.

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. For example, after a = 1; b = 1, a and b may or may not refer to the same object with the value one, depending on the implementation. This is because int{.interpreted-text role="class"} is an immutable type, so the reference to 1 can be reused. This behaviour depends on the implementation used, so should not be relied upon, but is something to be aware of when making use of object identity tests. However, after c = []; d = [], c and d are guaranteed to refer to two different, unique, newly created empty lists. (Note that e = f = [] assigns the same object to both e and f.)

The standard type hierarchy {#types}

::: index single: type pair: data; type pair: type; hierarchy pair: extension; module pair: C; language :::

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.), although such additions will often be provided via the standard library instead.

::: index single: attribute pair: special; attribute triple: generic; special; attribute :::

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.

None

::: index pair: object; None :::

This type has a single value. There is a single object with this value. This object is accessed through the built-in name None. 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.

NotImplemented

::: index pair: object; NotImplemented :::

This type has a single value. There is a single object with this value. This object is accessed through the built-in name NotImplemented{.interpreted-text role="data"}. Numeric methods and rich comparison methods should 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.) It should not be evaluated in a boolean context.

See implementing-the-arithmetic-operations{.interpreted-text role="ref"} for more details.

::: versionchanged 3.9 Evaluating NotImplemented{.interpreted-text role="data"} in a boolean context was deprecated. :::

::: versionchanged 3.14 Evaluating NotImplemented{.interpreted-text role="data"} in a boolean context now raises a TypeError{.interpreted-text role="exc"}. It previously evaluated to True{.interpreted-text role="const"} and emitted a DeprecationWarning{.interpreted-text role="exc"} since Python 3.9. :::

Ellipsis

::: index pair: object; Ellipsis single: ...; ellipsis literal :::

This type has a single value. There is a single object with this value. This object is accessed through the literal ... or the built-in name Ellipsis. Its truth value is true.

numbers.Number{.interpreted-text role="class"}

::: index pair: object; numeric :::

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.

The string representations of the numeric classes, computed by ~object.__repr__{.interpreted-text role="meth"} and ~object.__str__{.interpreted-text role="meth"}, have the following properties:

• They are valid numeric literals which, when passed to their class constructor, produce an object having the value of the original numeric.
• The representation is in base 10, when possible.
• Leading zeros, possibly excepting a single zero before a decimal point, are not shown.
• Trailing zeros, possibly excepting a single zero after a decimal point, are not shown.
• A sign is shown only when the number is negative.

Python distinguishes between integers, floating-point numbers, and complex numbers:

numbers.Integral{.interpreted-text role="class"}

::: index pair: object; integer :::

These represent elements from the mathematical set of integers (positive and negative).

:::: note ::: title Note :::

.. index:: pair: integer; representation

The rules for integer representation are intended to give the most meaningful interpretation of shift and mask operations involving negative integers. ::::

There are two types of integers:

Integers (int{.interpreted-text role="class"})

: 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.

Booleans (bool{.interpreted-text role="class"})

: ::: index pair: object; Boolean single: False single: True :::

These represent the truth values False and True. The two objects representing the values `False` and `True` are the only Boolean objects. The Boolean type is a subtype of the integer type, 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 `"False"` or `"True"` are returned, respectively.

numbers.Real{.interpreted-text role="class"} (float{.interpreted-text role="class"}) {#datamodel-float}

::: index pair: object; floating-point pair: floating-point; number pair: C; language pair: Java; language :::

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.

numbers.Complex{.interpreted-text role="class"} (complex{.interpreted-text role="class"})

::: index pair: object; complex pair: complex; number :::

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 z can be retrieved through the read-only attributes z.real and z.imag.

Sequences {#datamodel-sequences}

::: index pair: built-in function; len pair: object; sequence single: index operation single: item selection single: subscription :::

These represent finite ordered sets indexed by non-negative numbers. The built-in function len{.interpreted-text role="func"} returns the number of items of a sequence. When the length of a sequence is n, the index set contains the numbers 0, 1, ..., n-1. Item i of sequence a is selected by a[i]. Some sequences, including built-in sequences, interpret negative subscripts by adding the sequence length. For example, a[-2] equals a[n-2], the second to last item of sequence a with length n.

The resulting value must be a nonnegative integer less than the number of items in the sequence. If it is not, an IndexError{.interpreted-text role="exc"} is raised.

::: index single: slicing single: start (slice object attribute) single: stop (slice object attribute) single: step (slice object attribute) :::

Sequences also support slicing: a[start:stop] selects all items with index k such that start <= k < stop. When used as an expression, a slice is a sequence of the same type. The comment above about negative subscripts also applies to negative slice positions. Note that no error is raised if a slice position is less than zero or larger than the length of the sequence.

If start is missing or None{.interpreted-text role="data"}, slicing behaves as if start was zero. If stop is missing or None, slicing behaves as if stop was equal to the length of the sequence.

Some sequences also support "extended slicing" with a third "step" parameter: a[i:j:k] selects all items of a with index x where x = i + n*k, n >= 0 and i <= x < j.

Sequences are distinguished according to their mutability:

Immutable sequences

::: index pair: object; immutable sequence pair: object; immutable :::

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.)

The following types are immutable sequences:

::: index single: string; immutable sequences :::

Strings

: ::: index pair: built-in function; chr pair: built-in function; ord single: character pair: string; item single: Unicode :::

A string (`str`{.interpreted-text role="class"}) is a sequence of values that represent `characters`{.interpreted-text role="dfn"}, or more formally, *Unicode code points*. All the code points in the range `0` to `0x10FFFF` can be represented in a string.

Python doesn\'t have a dedicated *character* type. Instead, every code point in the string is represented as a string object with length `1`.

The built-in function `ord`{.interpreted-text role="func"} converts a code point from its string form to an integer in the range `0` to `0x10FFFF`; `chr`{.interpreted-text role="func"} converts an integer in the range `0` to `0x10FFFF` to the corresponding length `1` string object. `str.encode`{.interpreted-text role="meth"} can be used to convert a `str`{.interpreted-text role="class"} to `bytes`{.interpreted-text role="class"} using the given text encoding, and `bytes.decode`{.interpreted-text role="meth"} can be used to achieve the opposite.

Tuples

: ::: index pair: object; tuple pair: singleton; tuple pair: empty; tuple :::

The items of a `tuple`{.interpreted-text role="class"} 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.

Bytes

: ::: index bytes, byte :::

A `bytes`{.interpreted-text role="class"} object is an immutable array. The items are 8-bit bytes, represented by integers in the range 0 \<= x \< 256. Bytes literals (like `b'abc'`) and the built-in `bytes`{.interpreted-text role="func"} constructor can be used to create bytes objects. Also, bytes objects can be decoded to strings via the `~bytes.decode`{.interpreted-text role="meth"} method.

Mutable sequences

::: index pair: object; mutable sequence pair: object; mutable pair: assignment; statement single: subscription single: slicing :::

Mutable sequences can be changed after they are created. The subscription and slicing notations can be used as the target of assignment and del{.interpreted-text role="keyword"} (delete) statements.

:::: note ::: title Note :::

.. index:: pair: module; array .. index:: pair: module; collections

The collections{.interpreted-text role="mod"} and array{.interpreted-text role="mod"} module provide additional examples of mutable sequence types. ::::

There are currently two intrinsic mutable sequence types:

Lists

: ::: index pair: object; list :::

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.)

Byte Arrays

: ::: index bytearray :::

A bytearray object is a mutable array. They are created by the built-in `bytearray`{.interpreted-text role="func"} constructor. Aside from being mutable (and hence unhashable), byte arrays otherwise provide the same interface and functionality as immutable `bytes`{.interpreted-text role="class"} objects.

Set types

::: index pair: built-in function; len pair: object; set type :::

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 len{.interpreted-text role="func"} 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.

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., 1 and 1.0), only one of them can be contained in a set.

There are currently two intrinsic set types:

Sets

: ::: index pair: object; set :::

These represent a mutable set. They are created by the built-in `set`{.interpreted-text role="func"} constructor and can be modified afterwards by several methods, such as `~set.add`{.interpreted-text role="meth"}.

Frozen sets

: ::: index pair: object; frozenset :::

These represent an immutable set. They are created by the built-in `frozenset`{.interpreted-text role="func"} constructor. As a frozenset is immutable and `hashable`{.interpreted-text role="term"}, it can be used again as an element of another set, or as a dictionary key.

Mappings {#datamodel-mappings}

::: index pair: built-in function; len single: subscription pair: object; mapping :::

These represent finite sets of objects indexed by arbitrary index sets. The subscript notation a[k] selects the item indexed by k from the mapping a; this can be used in expressions and as the target of assignments or del{.interpreted-text role="keyword"} statements. The built-in function len{.interpreted-text role="func"} returns the number of items in a mapping.

There is currently a single intrinsic mapping type:

Dictionaries

::: index pair: object; dictionary :::

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., 1 and 1.0) then they can be used interchangeably to index the same dictionary entry.

Dictionaries preserve insertion order, meaning that keys will be produced in the same order they were added sequentially over the dictionary. Replacing an existing key does not change the order, however removing a key and re-inserting it will add it to the end instead of keeping its old place.

Dictionaries are mutable; they can be created by the {} notation (see section dict{.interpreted-text role="ref"}).

::: index pair: module; dbm.ndbm pair: module; dbm.gnu :::

The extension modules dbm.ndbm{.interpreted-text role="mod"} and dbm.gnu{.interpreted-text role="mod"} provide additional examples of mapping types, as does the collections{.interpreted-text role="mod"} module.

::: versionchanged 3.7 Dictionaries did not preserve insertion order in versions of Python before 3.6. In CPython 3.6, insertion order was preserved, but it was considered an implementation detail at that time rather than a language guarantee. :::

Callable types

::: index pair: object; callable pair: function; call single: invocation pair: function; argument :::

These are the types to which the function call operation (see section calls{.interpreted-text role="ref"}) can be applied:

User-defined functions {#user-defined-funcs}

::: index pair: user-defined; function pair: object; function pair: object; user-defined function :::

A user-defined function object is created by a function definition (see section function{.interpreted-text role="ref"}). It should be called with an argument list containing the same number of items as the function's formal parameter list.

Special read-only attributes

::: index single: __builtins__ (function attribute) single: __closure__ (function attribute) single: __globals__ (function attribute) pair: global; namespace :::

+---------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Attribute | Meaning | +===========================+===========================================================================================================================================================================================================================================================================+ | ::: attribute | A reference to the dictionary <dict>{.interpreted-text role="class"} that holds the function's builtins namespace. | | function.__builtins__ | | | ::: | ::: versionadded | | | 3.10 | | | ::: | +---------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A reference to the dictionary <dict>{.interpreted-text role="class"} that holds the function's global variables <naming>{.interpreted-text role="ref"} -- the global namespace of the module in which the function was defined. | | function.__globals__ | | | ::: | | +---------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | None or a tuple{.interpreted-text role="class"} of cells that contain bindings for the names specified in the ~codeobject.co_freevars{.interpreted-text role="attr"} attribute of the function's code object <function.__code__>{.interpreted-text role="attr"}. | | function.__closure__ | | | ::: | A cell object has the attribute cell_contents. This can be used to get the value of the cell, as well as set the value. | +---------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

Special writable attributes

::: index single: __doc__ (function attribute) single: __name__ (function attribute) single: __module__ (function attribute) single: __dict__ (function attribute) single: __defaults__ (function attribute) single: __code__ (function attribute) single: __annotations__ (function attribute) single: __annotate__ (function attribute) single: __kwdefaults__ (function attribute) single: __type_params__ (function attribute) :::

Most of these attributes check the type of the assigned value:

+------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Attribute | Meaning | +==============================+========================================================================================================================================================================================================================================================================================================================+ | ::: attribute | The function's documentation string, or None if unavailable. | | function.__doc__ | | | ::: | | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The function's name. See also: __name__ attributes <definition.__name__>{.interpreted-text role="attr"}. | | function.__name__ | | | ::: | | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The function's qualified name{.interpreted-text role="term"}. See also: __qualname__ attributes <definition.__qualname__>{.interpreted-text role="attr"}. | | function.__qualname__ | | | ::: | ::: versionadded | | | 3.3 | | | ::: | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The name of the module the function was defined in, or None if unavailable. | | function.__module__ | | | ::: | | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing default parameter{.interpreted-text role="term"} values for those parameters that have defaults, or None if no parameters have a default value. | | function.__defaults__ | | | ::: | | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The code object <code-objects>{.interpreted-text role="ref"} representing the compiled function body. | | function.__code__ | | | ::: | | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The namespace supporting arbitrary function attributes. See also: __dict__ attributes <object.__dict__>{.interpreted-text role="attr"}. | | function.__dict__ | | | ::: | | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A dictionary <dict>{.interpreted-text role="class"} containing annotations of parameters <parameter>{.interpreted-text role="term"}. The keys of the dictionary are the parameter names, and 'return' for the return annotation, if provided. See also: object.__annotations__{.interpreted-text role="attr"}. | | function.__annotations__ | | | ::: | ::: versionchanged | | | 3.14 Annotations are now lazily evaluated <lazy-evaluation>{.interpreted-text role="ref"}. See 649{.interpreted-text role="pep"}. | | | ::: | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The annotate function{.interpreted-text role="term"} for this function, or None if the function has no annotations. See object.__annotate__{.interpreted-text role="attr"}. | | function.__annotate__ | | | ::: | ::: versionadded | | | 3.14 | | | ::: | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A dictionary <dict>{.interpreted-text role="class"} containing defaults for keyword-only parameters <parameter>{.interpreted-text role="term"}. | | function.__kwdefaults__ | | | ::: | | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing the type parameters <type-params>{.interpreted-text role="ref"} of a generic function <generic-functions>{.interpreted-text role="ref"}. | | function.__type_params__ | | | ::: | ::: versionadded | | | 3.12 | | | ::: | +------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

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.

::: impl-detail CPython's current implementation only supports function attributes on user-defined functions. Function attributes on built-in functions <builtin-functions>{.interpreted-text role="ref"} may be supported in the future. :::

Additional information about a function's definition can be retrieved from its code object <code-objects>{.interpreted-text role="ref"} (accessible via the ~function.__code__{.interpreted-text role="attr"} attribute).

Instance methods

::: index pair: object; method pair: object; user-defined method pair: user-defined; method :::

An instance method object combines a class, a class instance and any callable object (normally a user-defined function).

::: index single: __func__ (method attribute) single: __self__ (method attribute) single: __doc__ (method attribute) single: __name__ (method attribute) single: __module__ (method attribute) :::

Special read-only attributes:

+-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | Refers to the class instance object to which the method is bound <method-binding>{.interpreted-text role="ref"} | | method.__self__ | | | ::: | | +-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | Refers to the original function object <user-defined-funcs>{.interpreted-text role="ref"} | | method.__func__ | | | ::: | | +-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The method's documentation (same as method.__func__.__doc__ <function.__doc__>{.interpreted-text role="attr"}). A string <str>{.interpreted-text role="class"} if the original function had a docstring, else None. | | method.__doc__ | | | ::: | | +-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The name of the method (same as method.__func__.__name__ <function.__name__>{.interpreted-text role="attr"}) | | method.__name__ | | | ::: | | +-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The name of the module the method was defined in, or None if unavailable. | | method.__module__ | | | ::: | | +-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

Methods also support accessing (but not setting) the arbitrary function attributes on the underlying function object <user-defined-funcs>{.interpreted-text role="ref"}.

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 <user-defined-funcs>{.interpreted-text role="ref"} or a classmethod{.interpreted-text role="class"} object.

::: {#method-binding} When an instance method object is created by retrieving a user-defined function object <user-defined-funcs>{.interpreted-text role="ref"} from a class via one of its instances, its ~method.__self__{.interpreted-text role="attr"} attribute is the instance, and the method object is said to be bound. The new method's ~method.__func__{.interpreted-text role="attr"} attribute is the original function object. :::

When an instance method object is created by retrieving a classmethod{.interpreted-text role="class"} object from a class or instance, its ~method.__self__{.interpreted-text role="attr"} attribute is the class itself, and its ~method.__func__{.interpreted-text role="attr"} attribute is the function object underlying the class method.

When an instance method object is called, the underlying function (~method.__func__{.interpreted-text role="attr"}) is called, inserting the class instance (~method.__self__{.interpreted-text role="attr"}) in front of the argument list. For instance, when !C{.interpreted-text role="class"} is a class which contains a definition for a function !f{.interpreted-text role="meth"}, and x is an instance of !C{.interpreted-text role="class"}, calling x.f(1) is equivalent to calling C.f(x, 1).

When an instance method object is derived from a classmethod{.interpreted-text role="class"} object, the "class instance" stored in ~method.__self__{.interpreted-text role="attr"} will actually be the class itself, so that calling either x.f(1) or C.f(1) is equivalent to calling f(C,1) where f is the underlying function.

It is important to note that user-defined functions which are attributes of a class instance are not converted to bound methods; this only happens when the function is an attribute of the class.

Generator functions

::: index single: generator; function single: generator; iterator :::

A function or method which uses the yield{.interpreted-text role="keyword"} statement (see section yield{.interpreted-text role="ref"}) is called a generator function{.interpreted-text role="dfn"}. Such a function, when called, always returns an iterator{.interpreted-text role="term"} object which can be used to execute the body of the function: calling the iterator's iterator.__next__{.interpreted-text role="meth"} method will cause the function to execute until it provides a value using the !yield{.interpreted-text role="keyword"} statement. When the function executes a return{.interpreted-text role="keyword"} statement or falls off the end, a StopIteration{.interpreted-text role="exc"} exception is raised and the iterator will have reached the end of the set of values to be returned.

Coroutine functions

::: index single: coroutine; function :::

A function or method which is defined using async def{.interpreted-text role="keyword"} is called a coroutine function{.interpreted-text role="dfn"}. Such a function, when called, returns a coroutine{.interpreted-text role="term"} object. It may contain await{.interpreted-text role="keyword"} expressions, as well as async with{.interpreted-text role="keyword"} and async for{.interpreted-text role="keyword"} statements. See also the coroutine-objects{.interpreted-text role="ref"} section.

Asynchronous generator functions

::: index single: asynchronous generator; function single: asynchronous generator; asynchronous iterator :::

A function or method which is defined using async def{.interpreted-text role="keyword"} and which uses the yield{.interpreted-text role="keyword"} statement is called a asynchronous generator function{.interpreted-text role="dfn"}. Such a function, when called, returns an asynchronous iterator{.interpreted-text role="term"} object which can be used in an async for{.interpreted-text role="keyword"} statement to execute the body of the function.

Calling the asynchronous iterator's aiterator.__anext__ <object.__anext__>{.interpreted-text role="meth"} method will return an awaitable{.interpreted-text role="term"} which when awaited will execute until it provides a value using the yield{.interpreted-text role="keyword"} expression. When the function executes an empty return{.interpreted-text role="keyword"} statement or falls off the end, a StopAsyncIteration{.interpreted-text role="exc"} exception is raised and the asynchronous iterator will have reached the end of the set of values to be yielded.

Built-in functions {#builtin-functions}

::: index pair: object; built-in function pair: object; function pair: C; language :::

A built-in function object is a wrapper around a C function. Examples of built-in functions are len{.interpreted-text role="func"} and math.sin{.interpreted-text role="func"} (math{.interpreted-text role="mod"} is a standard built-in module). The number and type of the arguments are determined by the C function. Special read-only attributes:

• !__doc__{.interpreted-text role="attr"} is the function's documentation string, or None if unavailable. See function.__doc__{.interpreted-text role="attr"}.
• !__name__{.interpreted-text role="attr"} is the function's name. See function.__name__{.interpreted-text role="attr"}.
• !__self__{.interpreted-text role="attr"} is set to None (but see the next item).
• !__module__{.interpreted-text role="attr"} is the name of the module the function was defined in or None if unavailable. See function.__module__{.interpreted-text role="attr"}.

Built-in methods {#builtin-methods}

::: index pair: object; built-in method pair: object; method pair: built-in; method :::

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 alist.append(), assuming alist is a list object. In this case, the special read-only attribute !__self__{.interpreted-text role="attr"} is set to the object denoted by alist. (The attribute has the same semantics as it does with other instance methods <method.__self__>{.interpreted-text role="attr"}.)

Classes

Classes are callable. These objects normally act as factories for new instances of themselves, but variations are possible for class types that override ~object.__new__{.interpreted-text role="meth"}. The arguments of the call are passed to !__new__{.interpreted-text role="meth"} and, in the typical case, to ~object.__init__{.interpreted-text role="meth"} to initialize the new instance.

Class Instances

Instances of arbitrary classes can be made callable by defining a ~object.__call__{.interpreted-text role="meth"} method in their class.

Modules {#module-objects}

::: index pair: statement; import pair: object; module :::

Modules are a basic organizational unit of Python code, and are created by the import system <importsystem>{.interpreted-text role="ref"} as invoked either by the import{.interpreted-text role="keyword"} statement, or by calling functions such as importlib.import_module{.interpreted-text role="func"} and built-in __import__{.interpreted-text role="func"}. A module object has a namespace implemented by a dictionary <dict>{.interpreted-text role="class"} object (this is the dictionary referenced by the ~function.__globals__{.interpreted-text role="attr"} attribute of functions defined in the module). Attribute references are translated to lookups in this dictionary, e.g., m.x is equivalent to m.__dict__["x"]. A module object does not contain the code object used to initialize the module (since it isn't needed once the initialization is done).

Attribute assignment updates the module's namespace dictionary, e.g., m.x = 1 is equivalent to m.__dict__["x"] = 1.

::: index single: __name__ (module attribute) single: __spec__ (module attribute) single: __package__ (module attribute) single: __loader__ (module attribute) single: __path__ (module attribute) single: __file__ (module attribute) single: __doc__ (module attribute) single: __annotations__ (module attribute) single: __annotate__ (module attribute) pair: module; namespace :::

Import-related attributes on module objects {#import-mod-attrs}

Module objects have the following attributes that relate to the import system <importsystem>{.interpreted-text role="ref"}. When a module is created using the machinery associated with the import system, these attributes are filled in based on the module's spec <module spec>{.interpreted-text role="term"}, before the loader{.interpreted-text role="term"} executes and loads the module.

To create a module dynamically rather than using the import system, it's recommended to use importlib.util.module_from_spec{.interpreted-text role="func"}, which will set the various import-controlled attributes to appropriate values. It's also possible to use the types.ModuleType{.interpreted-text role="class"} constructor to create modules directly, but this technique is more error-prone, as most attributes must be manually set on the module object after it has been created when using this approach.

::::: caution ::: title Caution :::

With the exception of ~module.__name__{.interpreted-text role="attr"}, it is strongly recommended that you rely on ~module.__spec__{.interpreted-text role="attr"} and its attributes instead of any of the other individual attributes listed in this subsection. Note that updating an attribute on !__spec__{.interpreted-text role="attr"} will not update the corresponding attribute on the module itself:

::: doctest >>> import typing >>> typing.__name__, typing.__spec__.name ('typing', 'typing') >>> typing.__spec__.name = 'spelling' >>> typing.__name__, typing.__spec__.name ('typing', 'spelling') >>> typing.__name__ = 'keyboard_smashing' >>> typing.__name__, typing.__spec__.name ('keyboard_smashing', 'spelling') ::: :::::

::: attribute module.__name__

The name used to uniquely identify the module in the import system. For a directly executed module, this will be set to "__main__".

This attribute must be set to the fully qualified name of the module. It is expected to match the value of module.__spec__.name <importlib.machinery.ModuleSpec.name>{.interpreted-text role="attr"}. :::

:::: attribute module.__spec__

A record of the module's import-system-related state.

Set to the module spec <importlib.machinery.ModuleSpec>{.interpreted-text role="class"} that was used when importing the module. See module-specs{.interpreted-text role="ref"} for more details.

::: versionadded 3.4 ::: ::::

:::::::: attribute module.__package__

The package{.interpreted-text role="term"} a module belongs to.

If the module is top-level (that is, not a part of any specific package) then the attribute should be set to '' (the empty string). Otherwise, it should be set to the name of the module's package (which can be equal to module.__name__{.interpreted-text role="attr"} if the module itself is a package). See 366{.interpreted-text role="pep"} for further details.

This attribute is used instead of ~module.__name__{.interpreted-text role="attr"} to calculate explicit relative imports for main modules. It defaults to None for modules created dynamically using the types.ModuleType{.interpreted-text role="class"} constructor; use importlib.util.module_from_spec{.interpreted-text role="func"} instead to ensure the attribute is set to a str{.interpreted-text role="class"}.

It is strongly recommended that you use module.__spec__.parent <importlib.machinery.ModuleSpec.parent>{.interpreted-text role="attr"} instead of !module.__package__{.interpreted-text role="attr"}. __package__{.interpreted-text role="attr"} is now only used as a fallback if !__spec__.parent{.interpreted-text role="attr"} is not set, and this fallback path is deprecated.

::: versionchanged 3.4 This attribute now defaults to None for modules created dynamically using the types.ModuleType{.interpreted-text role="class"} constructor. Previously the attribute was optional. :::

::: versionchanged 3.6 The value of !__package__{.interpreted-text role="attr"} is expected to be the same as __spec__.parent <importlib.machinery.ModuleSpec.parent>{.interpreted-text role="attr"}. __package__{.interpreted-text role="attr"} is now only used as a fallback during import resolution if !__spec__.parent{.interpreted-text role="attr"} is not defined. :::

::: versionchanged 3.10 ImportWarning{.interpreted-text role="exc"} is raised if an import resolution falls back to !__package__{.interpreted-text role="attr"} instead of __spec__.parent <importlib.machinery.ModuleSpec.parent>{.interpreted-text role="attr"}. :::

::: versionchanged 3.12 Raise DeprecationWarning{.interpreted-text role="exc"} instead of ImportWarning{.interpreted-text role="exc"} when falling back to !__package__{.interpreted-text role="attr"} during import resolution. :::

::: deprecated-removed 3.13 3.15 !__package__{.interpreted-text role="attr"} will cease to be set or taken into consideration by the import system or standard library. ::: ::::::::

::::: attribute module.__loader__

The loader{.interpreted-text role="term"} object that the import machinery used to load the module.

This attribute is mostly useful for introspection, but can be used for additional loader-specific functionality, for example getting data associated with a loader.

!__loader__{.interpreted-text role="attr"} defaults to None for modules created dynamically using the types.ModuleType{.interpreted-text role="class"} constructor; use importlib.util.module_from_spec{.interpreted-text role="func"} instead to ensure the attribute is set to a loader{.interpreted-text role="term"} object.

It is strongly recommended that you use module.__spec__.loader <importlib.machinery.ModuleSpec.loader>{.interpreted-text role="attr"} instead of !module.__loader__{.interpreted-text role="attr"}.

::: versionchanged 3.4 This attribute now defaults to None for modules created dynamically using the types.ModuleType{.interpreted-text role="class"} constructor. Previously the attribute was optional. :::

::: deprecated-removed 3.12 3.16 Setting !__loader__{.interpreted-text role="attr"} on a module while failing to set !__spec__.loader{.interpreted-text role="attr"} is deprecated. In Python 3.16, !__loader__{.interpreted-text role="attr"} will cease to be set or taken into consideration by the import system or the standard library. ::: :::::

::: attribute module.__path__

A (possibly empty) sequence{.interpreted-text role="term"} of strings enumerating the locations where the package's submodules will be found. Non-package modules should not have a !__path__{.interpreted-text role="attr"} attribute. See package-path-rules{.interpreted-text role="ref"} for more details.

It is strongly recommended that you use module.__spec__.submodule_search_locations <importlib.machinery.ModuleSpec.submodule_search_locations>{.interpreted-text role="attr"} instead of !module.__path__{.interpreted-text role="attr"}. :::

::::: attribute module.__file__

!__file__{.interpreted-text role="attr"} is an optional attribute that may or may not be set. Both attributes should be a str{.interpreted-text role="class"} when they are available.

An optional attribute, !__file__{.interpreted-text role="attr"} indicates the pathname of the file from which the module was loaded (if loaded from a file), or the pathname of the shared library file for extension modules loaded dynamically from a shared library. It might be missing for certain types of modules, such as C modules that are statically linked into the interpreter, and the import system <importsystem>{.interpreted-text role="ref"} may opt to leave it unset if it has no semantic meaning (for example, a module loaded from a database).

::: deprecated-removed 3.13 3.15 Setting __cached__ on a module while failing to set !__spec__.cached{.interpreted-text role="attr"} is deprecated. In Python 3.15, __cached__ will cease to be set or taken into consideration by the import system or standard library. :::

::: versionchanged 3.15 __cached__ is no longer set. ::: :::::

Other writable attributes on module objects

As well as the import-related attributes listed above, module objects also have the following writable attributes:

::: attribute module.__doc__

The module's documentation string, or None if unavailable. See also: __doc__ attributes <definition.__doc__>{.interpreted-text role="attr"}. :::

:::: attribute module.__annotations__

A dictionary containing variable annotations <variable annotation>{.interpreted-text role="term"} collected during module body execution. For best practices on working with !__annotations__{.interpreted-text role="attr"}, see annotationlib{.interpreted-text role="mod"}.

::: versionchanged 3.14 Annotations are now lazily evaluated <lazy-evaluation>{.interpreted-text role="ref"}. See 649{.interpreted-text role="pep"}. ::: ::::

:::: attribute module.__annotate__

The annotate function{.interpreted-text role="term"} for this module, or None if the module has no annotations. See also: ~object.__annotate__{.interpreted-text role="attr"} attributes.

::: versionadded 3.14 ::: ::::

Module dictionaries

Module objects also have the following special read-only attribute:

::: index single: __dict__ (module attribute) :::

:::: attribute module.__dict__

The module's namespace as a dictionary object. Uniquely among the attributes listed here, !__dict__{.interpreted-text role="attr"} cannot be accessed as a global variable from within a module; it can only be accessed as an attribute on module objects.

::: impl-detail 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. ::: ::::

Custom classes {#class-attrs-and-methods}

Custom class types are typically created by class definitions (see section class{.interpreted-text role="ref"}). A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g., C.x is translated to C.__dict__["x"] (although 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. This search of the base classes uses the 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 Python can be found at python_2.3_mro{.interpreted-text role="ref"}.

::: index pair: object; class pair: object; class instance pair: object; instance pair: class object; call single: container pair: object; dictionary pair: class; attribute :::

When a class attribute reference (for class !C{.interpreted-text role="class"}, say) would yield a class method object, it is transformed into an instance method object whose ~method.__self__{.interpreted-text role="attr"} attribute is !C{.interpreted-text role="class"}. When it would yield a staticmethod{.interpreted-text role="class"} object, it is transformed into the object wrapped by the static method object. See section descriptors{.interpreted-text role="ref"} for another way in which attributes retrieved from a class may differ from those actually contained in its ~object.__dict__{.interpreted-text role="attr"}.

::: index triple: class; attribute; assignment :::

Class attribute assignments update the class's dictionary, never the dictionary of a base class.

::: index pair: class object; call :::

A class object can be called (see above) to yield a class instance (see below).

Special attributes

::: index single: __name__ (class attribute) single: __module__ (class attribute) single: __dict__ (class attribute) single: __bases__ (class attribute) single: __base__ (class attribute) single: __doc__ (class attribute) single: __annotations__ (class attribute) single: __annotate__ (class attribute) single: __type_params__ (class attribute) single: __static_attributes__ (class attribute) single: __firstlineno__ (class attribute) :::

+--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Attribute | Meaning | +================================+=====================================================================================================================================================================================================================================================================================================================================================================+ | ::: attribute | The class's name. See also: __name__ attributes <definition.__name__>{.interpreted-text role="attr"}. | | type.__name__ | | | ::: | | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The class's qualified name{.interpreted-text role="term"}. See also: __qualname__ attributes <definition.__qualname__>{.interpreted-text role="attr"}. | | type.__qualname__ | | | ::: | | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The name of the module in which the class was defined. | | type.__module__ | | | ::: | | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A mapping proxy <types.MappingProxyType>{.interpreted-text role="class"} providing a read-only view of the class's namespace. See also: __dict__ attributes <object.__dict__>{.interpreted-text role="attr"}. | | type.__dict__ | | | ::: | | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing the class's bases. In most cases, for a class defined as class X(A, B, C), X.__bases__ will be exactly equal to (A, B, C). | | type.__bases__ | | | ::: | | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | ::: impl-detail | | type.__base__ | The single base class in the inheritance chain that is responsible for the memory layout of instances. This attribute corresponds to ~PyTypeObject.tp_base{.interpreted-text role="c:member"} at the C level. | | ::: | ::: | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The class's documentation string, or None if undefined. Not inherited by subclasses. | | type.__doc__ | | | ::: | | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A dictionary containing variable annotations <variable annotation>{.interpreted-text role="term"} collected during class body execution. See also: __annotations__ attributes <object.__annotations__>{.interpreted-text role="attr"}. | | type.__annotations__ | | | ::: | For best practices on working with ~object.__annotations__{.interpreted-text role="attr"}, please see annotationlib{.interpreted-text role="mod"}. Use annotationlib.get_annotations{.interpreted-text role="func"} instead of accessing this attribute directly. | | | | | | :::: warning | | | ::: title | | | Warning | | | ::: | | | | | | Accessing the !__annotations__{.interpreted-text role="attr"} attribute directly on a class object may return annotations for the wrong class, specifically in certain cases where the class, its base class, or a metaclass is defined under from __future__ import annotations. See 749 <749#pep749-metaclasses>{.interpreted-text role="pep"} for details. | | | | | | This attribute does not exist on certain builtin classes. On user-defined classes without __annotations__, it is an empty dictionary. | | | :::: | | | | | | ::: versionchanged | | | 3.14 Annotations are now lazily evaluated <lazy-evaluation>{.interpreted-text role="ref"}. See 649{.interpreted-text role="pep"}. | | | ::: | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: method | The annotate function{.interpreted-text role="term"} for this class, or None if the class has no annotations. See also: __annotate__ attributes <object.__annotate__>{.interpreted-text role="attr"}. | | type.__annotate__ | | | ::: | ::: versionadded | | | 3.14 | | | ::: | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing the type parameters <type-params>{.interpreted-text role="ref"} of a generic class <generic-classes>{.interpreted-text role="ref"}. | | type.__type_params__ | | | ::: | ::: versionadded | | | 3.12 | | | ::: | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing names of attributes of this class which are assigned through self.X from any function in its body. | | type.__static_attributes__ | | | ::: | ::: versionadded | | | 3.13 | | | ::: | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The line number of the first line of the class definition, including decorators. Setting the ~type.__module__{.interpreted-text role="attr"} attribute removes the !__firstlineno__{.interpreted-text role="attr"} item from the type's dictionary. | | type.__firstlineno__ | | | ::: | ::: versionadded | | | 3.13 | | | ::: | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The tuple{.interpreted-text role="class"} of classes that are considered when looking for base classes during method resolution. | | type.__mro__ | | | ::: | | +--------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

Special methods

In addition to the special attributes described above, all Python classes also have the following two methods available:

::: method type.mro

This method can be overridden by a metaclass to customize the method resolution order for its instances. It is called at class instantiation, and its result is stored in ~type.__mro__{.interpreted-text role="attr"}. :::

:::: method type.__subclasses__

Each class keeps a list of weak references to its immediate subclasses. This method returns a list of all those references still alive. The list is in definition order. Example:

::: doctest >>> class A: pass >>> class B(A): pass >>> A.__subclasses__() [<class 'B'>] ::: ::::

Class instances

::: index pair: object; class instance pair: object; instance pair: class; instance pair: class instance; attribute :::

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, it is transformed into an instance method object whose ~method.__self__{.interpreted-text role="attr"} attribute is the instance. Static method and class method objects are also transformed; see above under "Classes". See section descriptors{.interpreted-text role="ref"} for another way in which attributes of a class retrieved via its instances may differ from the objects actually stored in the class's ~object.__dict__{.interpreted-text role="attr"}. If no class attribute is found, and the object's class has a ~object.__getattr__{.interpreted-text role="meth"} method, that is called to satisfy the lookup.

::: index triple: class instance; attribute; assignment :::

Attribute assignments and deletions update the instance's dictionary, never a class's dictionary. If the class has a ~object.__setattr__{.interpreted-text role="meth"} or ~object.__delattr__{.interpreted-text role="meth"} method, this is called instead of updating the instance dictionary directly.

::: index pair: object; numeric pair: object; sequence pair: object; mapping :::

Class instances can pretend to be numbers, sequences, or mappings if they have methods with certain special names. See section specialnames{.interpreted-text role="ref"}.

Special attributes

::: index single: __dict__ (instance attribute) single: __class__ (instance attribute) :::

::: attribute object.__class__

The class to which a class instance belongs. :::

::: attribute object.__dict__

A dictionary or other mapping object used to store an object's (writable) attributes. Not all instances have a !__dict__{.interpreted-text role="attr"} attribute; see the section on slots{.interpreted-text role="ref"} for more details. :::

I/O objects (also known as file objects)

::: index pair: built-in function; open pair: module; io single: popen() (in module os) single: makefile() (socket method) single: sys.stdin single: sys.stdout single: sys.stderr single: stdio single: stdin (in module sys) single: stdout (in module sys) single: stderr (in module sys) :::

A file object{.interpreted-text role="term"} represents an open file. Various shortcuts are available to create file objects: the open{.interpreted-text role="func"} built-in function, and also os.popen{.interpreted-text role="func"}, os.fdopen{.interpreted-text role="func"}, and the ~socket.socket.makefile{.interpreted-text role="meth"} method of socket objects (and perhaps by other functions or methods provided by extension modules).

The objects sys.stdin, sys.stdout and sys.stderr are initialized to file objects corresponding to the interpreter's standard input, output and error streams; they are all open in text mode and therefore follow the interface defined by the io.TextIOBase{.interpreted-text role="class"} abstract class.

Internal types

::: index single: internal type single: types, internal :::

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.

Code objects

::: index bytecode, object; code, code object :::

Code objects represent byte-compiled executable Python code, or bytecode{.interpreted-text role="term"}. 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.

::: index single: co_argcount (code object attribute) single: co_posonlyargcount (code object attribute) single: co_kwonlyargcount (code object attribute) single: co_code (code object attribute) single: co_consts (code object attribute) single: co_filename (code object attribute) single: co_firstlineno (code object attribute) single: co_flags (code object attribute) single: co_lnotab (code object attribute) single: co_name (code object attribute) single: co_names (code object attribute) single: co_nlocals (code object attribute) single: co_stacksize (code object attribute) single: co_varnames (code object attribute) single: co_cellvars (code object attribute) single: co_freevars (code object attribute) single: co_qualname (code object attribute) :::

Special read-only attributes

+-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The function name | | codeobject.co_name | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The fully qualified function name | | codeobject.co_qualname | | | ::: | ::: versionadded | | | 3.11 | | | ::: | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The total number of positional parameters <parameter>{.interpreted-text role="term"} (including positional-only parameters and parameters with default values) that the function has | | codeobject.co_argcount | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The number of positional-only parameters <parameter>{.interpreted-text role="term"} (including arguments with default values) that the function has | | codeobject.co_posonlyargcount | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The number of keyword-only parameters <parameter>{.interpreted-text role="term"} (including arguments with default values) that the function has | | codeobject.co_kwonlyargcount | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The number of local variables <naming>{.interpreted-text role="ref"} used by the function (including parameters) | | codeobject.co_nlocals | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing the names of the local variables in the function (starting with the parameter names) | | codeobject.co_varnames | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing the names of local variables <naming>{.interpreted-text role="ref"} that are referenced from at least one nested scope{.interpreted-text role="term"} inside the function | | codeobject.co_cellvars | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing the names of free (closure) variables <closure variable>{.interpreted-text role="term"} that a nested scope{.interpreted-text role="term"} references in an outer scope. See also function.__closure__{.interpreted-text role="attr"}. | | codeobject.co_freevars | | | ::: | Note: references to global and builtin names are not included. | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A string representing the sequence of bytecode{.interpreted-text role="term"} instructions in the function | | codeobject.co_code | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing the literals used by the bytecode{.interpreted-text role="term"} in the function | | codeobject.co_consts | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A tuple{.interpreted-text role="class"} containing the names used by the bytecode{.interpreted-text role="term"} in the function | | codeobject.co_names | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The name of the file from which the code was compiled | | codeobject.co_filename | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The line number of the first line of the function | | codeobject.co_firstlineno | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | A string encoding the mapping from bytecode{.interpreted-text role="term"} offsets to line numbers. For details, see the source code of the interpreter. | | codeobject.co_lnotab | | | ::: | ::: deprecated | | | 3.12 This attribute of code objects is deprecated, and may be removed in Python 3.15. | | | ::: | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The required stack size of the code object | | codeobject.co_stacksize | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | An integer <int>{.interpreted-text role="class"} encoding a number of flags for the interpreter. | | codeobject.co_flags | | | ::: | | +-------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

::: index pair: object; generator :::

The following flag bits are defined for ~codeobject.co_flags{.interpreted-text role="attr"}: bit 0x04 is set if the function uses the *arguments syntax to accept an arbitrary number of positional arguments; bit 0x08 is set if the function uses the **keywords syntax to accept arbitrary keyword arguments; bit 0x20 is set if the function is a generator. See inspect-module-co-flags{.interpreted-text role="ref"} for details on the semantics of each flags that might be present.

Future feature declarations (for example, from __future__ import division) also use bits in ~codeobject.co_flags{.interpreted-text role="attr"} to indicate whether a code object was compiled with a particular feature enabled. See ~__future__._Feature.compiler_flag{.interpreted-text role="attr"}.

Other bits in ~codeobject.co_flags{.interpreted-text role="attr"} are reserved for internal use.

::: index single: documentation string :::

If a code object represents a function and has a docstring, the ~inspect.CO_HAS_DOCSTRING{.interpreted-text role="data"} bit is set in ~codeobject.co_flags{.interpreted-text role="attr"} and the first item in ~codeobject.co_consts{.interpreted-text role="attr"} is the docstring of the function.

Methods on code objects

:::::: method codeobject.co_positions()

Returns an iterable over the source code positions of each bytecode{.interpreted-text role="term"} instruction in the code object.

The iterator returns tuple{.interpreted-text role="class"}s containing the (start_line, end_line, start_column, end_column). The i-th tuple corresponds to the position of the source code that compiled to the i-th code unit. Column information is 0-indexed utf-8 byte offsets on the given source line.

This positional information can be missing. A non-exhaustive lists of cases where this may happen:

• Running the interpreter with -X{.interpreted-text role="option"} no_debug_ranges.
• Loading a pyc file compiled while using -X{.interpreted-text role="option"} no_debug_ranges.
• Position tuples corresponding to artificial instructions.
• Line and column numbers that can't be represented due to implementation specific limitations.

When this occurs, some or all of the tuple elements can be None{.interpreted-text role="const"}.

::: versionadded 3.11 :::

:::: note ::: title Note :::

This feature requires storing column positions in code objects which may result in a small increase of disk usage of compiled Python files or interpreter memory usage. To avoid storing the extra information and/or deactivate printing the extra traceback information, the -X{.interpreted-text role="option"} no_debug_ranges command line flag or the PYTHONNODEBUGRANGES{.interpreted-text role="envvar"} environment variable can be used. :::: ::::::

::::: method codeobject.co_lines()

Returns an iterator that yields information about successive ranges of bytecode{.interpreted-text role="term"}s. Each item yielded is a (start, end, lineno) tuple{.interpreted-text role="class"}:

• start (an int{.interpreted-text role="class"}) represents the offset (inclusive) of the start of the bytecode{.interpreted-text role="term"} range
• end (an int{.interpreted-text role="class"}) represents the offset (exclusive) of the end of the bytecode{.interpreted-text role="term"} range
• lineno is an int{.interpreted-text role="class"} representing the line number of the bytecode{.interpreted-text role="term"} range, or None if the bytecodes in the given range have no line number

The items yielded will have the following properties:

• The first range yielded will have a start of 0.
• The (start, end) ranges will be non-decreasing and consecutive. That is, for any pair of tuple{.interpreted-text role="class"}s, the start of the second will be equal to the end of the first.
• No range will be backwards: end >= start for all triples.
• The last tuple{.interpreted-text role="class"} yielded will have end equal to the size of the bytecode{.interpreted-text role="term"}.

Zero-width ranges, where start == end, are allowed. Zero-width ranges are used for lines that are present in the source code, but have been eliminated by the bytecode{.interpreted-text role="term"} compiler.

::: versionadded 3.10 :::

::: seealso

626{.interpreted-text role="pep"} - Precise line numbers for debugging and other tools.

: The PEP that introduced the !co_lines{.interpreted-text role="meth"} method. ::: :::::

:::: method codeobject.replace(**kwargs)

Return a copy of the code object with new values for the specified fields.

Code objects are also supported by the generic function copy.replace{.interpreted-text role="func"}.

::: versionadded 3.8 ::: ::::

Frame objects

::: index pair: object; frame :::

Frame objects represent execution frames. They may occur in traceback objects <traceback-objects>{.interpreted-text role="ref"}, and are also passed to registered trace functions.

::: index single: f_back (frame attribute) single: f_code (frame attribute) single: f_globals (frame attribute) single: f_locals (frame attribute) single: f_lasti (frame attribute) single: f_builtins (frame attribute) single: f_generator (frame attribute) :::

Special read-only attributes

+-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | Points to the previous stack frame (towards the caller), or None if this is the bottom stack frame | | frame.f_back | | | ::: | | +-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The code object <code-objects>{.interpreted-text role="ref"} being executed in this frame. Accessing this attribute raises an auditing event <auditing>{.interpreted-text role="ref"} object.__getattr__ with arguments obj and "f_code". | | frame.f_code | | | ::: | | +-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The mapping used by the frame to look up local variables <naming>{.interpreted-text role="ref"}. If the frame refers to an optimized scope{.interpreted-text role="term"}, this may return a write-through proxy object. | | frame.f_locals | | | ::: | ::: versionchanged | | | 3.13 Return a proxy for optimized scopes. | | | ::: | +-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The dictionary used by the frame to look up global variables <naming>{.interpreted-text role="ref"} | | frame.f_globals | | | ::: | | +-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The dictionary used by the frame to look up built-in (intrinsic) names <naming>{.interpreted-text role="ref"} | | frame.f_builtins | | | ::: | | +-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The "precise instruction" of the frame object (this is an index into the bytecode{.interpreted-text role="term"} string of the code object <code-objects>{.interpreted-text role="ref"}) | | frame.f_lasti | | | ::: | | +-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | The generator{.interpreted-text role="term"} or coroutine{.interpreted-text role="term"} object that owns this frame, or None if the frame is a normal function. | | frame.f_generator | | | ::: | ::: versionadded | | | 3.14 | | | ::: | +-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

::: index single: f_trace (frame attribute) single: f_trace_lines (frame attribute) single: f_trace_opcodes (frame attribute) single: f_lineno (frame attribute) :::

Special writable attributes

+-----------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | If not None, this is a function called for various events during code execution (this is used by debuggers). Normally an event is triggered for each new source line (see ~frame.f_trace_lines{.interpreted-text role="attr"}). | | frame.f_trace | | | ::: | | +-----------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | Set this attribute to False{.interpreted-text role="const"} to disable triggering a tracing event for each source line. | | frame.f_trace_lines | | | ::: | | +-----------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | Set this attribute to True{.interpreted-text role="const"} to allow per-opcode events to be requested. Note that this may lead to undefined interpreter behaviour if exceptions raised by the trace function escape to the function being traced. | | frame.f_trace_opcodes | | | ::: | | +-----------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | 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 this attribute. | | frame.f_lineno | | | ::: | | +-----------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

Frame object methods

Frame objects support one method:

::::: method frame.clear()

This method clears all references to local variables <naming>{.interpreted-text role="ref"} held by the frame. Also, if the frame belonged to a generator{.interpreted-text role="term"}, the generator is finalized. This helps break reference cycles involving frame objects (for example when catching an exception <bltin-exceptions>{.interpreted-text role="ref"} and storing its traceback <traceback-objects>{.interpreted-text role="ref"} for later use).

RuntimeError{.interpreted-text role="exc"} is raised if the frame is currently executing or suspended.

::: versionadded 3.4 :::

::: versionchanged 3.13 Attempting to clear a suspended frame raises RuntimeError{.interpreted-text role="exc"} (as has always been the case for executing frames). ::: :::::

Traceback objects

::: index pair: object; traceback pair: stack; trace pair: exception; handler pair: execution; stack single: exc_info (in module sys) single: last_traceback (in module sys) single: sys.exc_info single: sys.exception single: sys.last_traceback :::

Traceback objects represent the stack trace of an exception <tut-errors>{.interpreted-text role="ref"}. A traceback object is implicitly created when an exception occurs, and may also be explicitly created by calling types.TracebackType{.interpreted-text role="class"}.

::: versionchanged 3.7 Traceback objects can now be explicitly instantiated from Python code. :::

For implicitly created tracebacks, 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 try{.interpreted-text role="ref"}.) It is accessible as the third item of the tuple returned by sys.exc_info{.interpreted-text role="func"}, and as the ~BaseException.__traceback__{.interpreted-text role="attr"} attribute of the caught exception.

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 sys.last_traceback{.interpreted-text role="data"}.

For explicitly created tracebacks, it is up to the creator of the traceback to determine how the ~traceback.tb_next{.interpreted-text role="attr"} attributes should be linked to form a full stack trace.

::: index single: tb_frame (traceback attribute) single: tb_lineno (traceback attribute) single: tb_lasti (traceback attribute) pair: statement; try :::

Special read-only attributes:

+---------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | Points to the execution frame <frame-objects>{.interpreted-text role="ref"} of the current level. | | traceback.tb_frame | | | ::: | Accessing this attribute raises an auditing event <auditing>{.interpreted-text role="ref"} object.__getattr__ with arguments obj and "tb_frame". | +---------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | Gives the line number where the exception occurred | | traceback.tb_lineno | | | ::: | | +---------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------+ | ::: attribute | Indicates the "precise instruction". | | traceback.tb_lasti | | | ::: | | +---------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------+

The line number and last instruction in the traceback may differ from the line number of its frame object <frame-objects>{.interpreted-text role="ref"} if the exception occurred in a try{.interpreted-text role="keyword"} statement with no matching except clause or with a finally{.interpreted-text role="keyword"} clause.

::: index single: tb_next (traceback attribute) :::

:::: attribute traceback.tb_next

The special writable attribute !tb_next{.interpreted-text role="attr"} is the next level in the stack trace (towards the frame where the exception occurred), or None if there is no next level.

::: versionchanged 3.7 This attribute is now writable ::: ::::

Slice objects

::: index pair: built-in function; slice :::

Slice objects are used to represent slices for ~object.__getitem__{.interpreted-text role="meth"} methods. They are also created by the built-in slice{.interpreted-text role="func"} function.

::: versionadded 3.15

The slice{.interpreted-text role="func"} type now supports subscription <subscriptions>{.interpreted-text role="ref"}. For example, slice[float] may be used in type annotations to indicate a slice containing float{.interpreted-text role="type"} objects. :::

::: index single: start (slice object attribute) single: stop (slice object attribute) single: step (slice object attribute) :::

Special read-only attributes: ~slice.start{.interpreted-text role="attr"} is the lower bound; ~slice.stop{.interpreted-text role="attr"} is the upper bound; ~slice.step{.interpreted-text role="attr"} is the step value; each is None if omitted. These attributes can have any type.

Slice objects support one method:

::: method slice.indices(self, length)

This method takes a single integer argument length and computes information about the slice that the slice object would describe if applied to a sequence of length items. It returns a tuple of three integers; respectively these are the start and stop indices and the step or stride length of the slice. Missing or out-of-bounds indices are handled in a manner consistent with regular slices. :::

Static method objects

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 also callable. Static method objects are created by the built-in staticmethod{.interpreted-text role="func"} constructor.

Class method objects

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 "instance methods" <instance-methods>{.interpreted-text role="ref"}. Class method objects are created by the built-in classmethod{.interpreted-text role="func"} constructor.

Special method names {#specialnames}

::: index pair: operator; overloading single: __getitem__() (mapping object method) :::

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 operator overloading{.interpreted-text role="dfn"}, allowing classes to define their own behavior with respect to language operators. For instance, if a class defines a method named ~object.__getitem__{.interpreted-text role="meth"}, and x is an instance of this class, then x[i] is roughly equivalent to type(x).__getitem__(x, i). Except where mentioned, attempts to execute an operation raise an exception when no appropriate method is defined (typically AttributeError{.interpreted-text role="exc"} or TypeError{.interpreted-text role="exc"}).

Setting a special method to None indicates that the corresponding operation is not available. For example, if a class sets ~object.__iter__{.interpreted-text role="meth"} to None, the class is not iterable, so calling iter{.interpreted-text role="func"} on its instances will raise a TypeError{.interpreted-text role="exc"} (without falling back to ~object.__getitem__{.interpreted-text role="meth"}).[^2]

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 NodeList <dom-nodelist-objects>{.interpreted-text role="ref"} interface in the W3C's Document Object Model.)

Basic customization {#customization}

:::: method object.__new__(cls[, ...])

::: index pair: subclassing; immutable types :::

Called to create a new instance of class cls. __new__{.interpreted-text role="meth"} 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 __new__{.interpreted-text role="meth"} should be the new object instance (usually an instance of cls).

Typical implementations create a new instance of the class by invoking the superclass's __new__{.interpreted-text role="meth"} method using super().__new__(cls[, ...]) with appropriate arguments and then modifying the newly created instance as necessary before returning it.

If __new__{.interpreted-text role="meth"} is invoked during object construction and it returns an instance of cls, then the new instance's __init__{.interpreted-text role="meth"} method will be invoked like __init__(self[, ...]), where self is the new instance and the remaining arguments are the same as were passed to the object constructor.

If __new__{.interpreted-text role="meth"} does not return an instance of cls, then the new instance's __init__{.interpreted-text role="meth"} method will not be invoked.

__new__{.interpreted-text role="meth"} 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. ::::

:::: method object.__init__(self[, ...])

::: index pair: class; constructor :::

Called after the instance has been created (by __new__{.interpreted-text role="meth"}), but before it is returned to the caller. The arguments are those passed to the class constructor expression. If a base class has an __init__{.interpreted-text role="meth"} method, the derived class's __init__{.interpreted-text role="meth"} method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: super().__init__([args...]).

Because __new__{.interpreted-text role="meth"} and __init__{.interpreted-text role="meth"} work together in constructing objects (__new__{.interpreted-text role="meth"} to create it, and __init__{.interpreted-text role="meth"} to customize it), no non-None value may be returned by __init__{.interpreted-text role="meth"}; doing so will cause a TypeError{.interpreted-text role="exc"} to be raised at runtime. ::::

::::::::::: method object.__del__(self)

::: index single: destructor single: finalizer pair: statement; del :::

Called when the instance is about to be destroyed. This is also called a finalizer or (improperly) a destructor. If a base class has a __del__{.interpreted-text role="meth"} method, the derived class's __del__{.interpreted-text role="meth"} method, if any, must explicitly call it to ensure proper deletion of the base class part of the instance.

It is possible (though not recommended!) for the __del__{.interpreted-text role="meth"} method to postpone destruction of the instance by creating a new reference to it. This is called object resurrection. It is implementation-dependent whether __del__{.interpreted-text role="meth"} is called a second time when a resurrected object is about to be destroyed; the current CPython{.interpreted-text role="term"} implementation only calls it once.

It is not guaranteed that __del__{.interpreted-text role="meth"} methods are called for objects that still exist when the interpreter exits. weakref.finalize{.interpreted-text role="class"} provides a straightforward way to register a cleanup function to be called when an object is garbage collected.

:::: note ::: title Note :::

del x doesn't directly call x.__del__() --- the former decrements the reference count for x by one, and the latter is only called when x's reference count reaches zero. ::::

:::: impl-detail It is possible for a reference cycle to prevent the reference count of an object from going to zero. In this case, the cycle will be later detected and deleted by the cyclic garbage collector <garbage collection>{.interpreted-text role="term"}. A common cause of reference cycles is when an exception has been caught in a local variable. The frame's locals then reference the exception, which references its own traceback, which references the locals of all frames caught in the traceback.

::: seealso Documentation for the gc{.interpreted-text role="mod"} module. ::: ::::

:::: warning ::: title Warning :::

Due to the precarious circumstances under which __del__{.interpreted-text role="meth"} methods are invoked, exceptions that occur during their execution are ignored, and a warning is printed to sys.stderr instead. In particular:

• __del__{.interpreted-text role="meth"} can be invoked when arbitrary code is being executed, including from any arbitrary thread. If __del__{.interpreted-text role="meth"} needs to take a lock or invoke any other blocking resource, it may deadlock as the resource may already be taken by the code that gets interrupted to execute __del__{.interpreted-text role="meth"}.
• __del__{.interpreted-text role="meth"} can be executed during interpreter shutdown. As a consequence, the global variables it needs to access (including other modules) may already have been deleted or set to None. 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 __del__{.interpreted-text role="meth"} method is called. ::::

::: index single: repr() (built-in function); __repr__() (object method) ::: :::::::::::

:::: method object.__repr__(self)

Called by the repr{.interpreted-text role="func"} built-in function 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 <...some useful description...> should be returned. The return value must be a string object. If a class defines __repr__{.interpreted-text role="meth"} but not __str__{.interpreted-text role="meth"}, then __repr__{.interpreted-text role="meth"} is also used when an "informal" string representation of instances of that class is required.

This is typically used for debugging, so it is important that the representation is information-rich and unambiguous. A default implementation is provided by the object{.interpreted-text role="class"} class itself.

::: index single: string; __str__() (object method) single: format() (built-in function); __str__() (object method) single: print() (built-in function); __str__() (object method) ::: ::::

::: method object.__str__(self)

Called by str(object) <str>{.interpreted-text role="func"}, the default __format__{.interpreted-text role="meth"} implementation, and the built-in function print{.interpreted-text role="func"}, to compute the "informal" or nicely printable string representation of an object. The return value must be a str <textseq>{.interpreted-text role="ref"} object.

This method differs from object.__repr__{.interpreted-text role="meth"} in that there is no expectation that __str__{.interpreted-text role="meth"} return a valid Python expression: a more convenient or concise representation can be used.

The default implementation defined by the built-in type object{.interpreted-text role="class"} calls object.__repr__{.interpreted-text role="meth"}. :::

::::: method object.__bytes__(self)

::: index pair: built-in function; bytes :::

Called by bytes <func-bytes>{.interpreted-text role="ref"} to compute a byte-string representation of an object. This should return a bytes{.interpreted-text role="class"} object. The object{.interpreted-text role="class"} class itself does not provide this method.

::: index single: string; __format__() (object method) pair: string; conversion pair: built-in function; print ::: :::::

::::: method object.__format__(self, format_spec)

Called by the format{.interpreted-text role="func"} built-in function, and by extension, evaluation of formatted string literals <f-strings>{.interpreted-text role="ref"} and the str.format{.interpreted-text role="meth"} method, to produce a "formatted" string representation of an object. The format_spec argument is a string that contains a description of the formatting options desired. The interpretation of the format_spec argument is up to the type implementing __format__{.interpreted-text role="meth"}, however most classes will either delegate formatting to one of the built-in types, or use a similar formatting option syntax.

See formatspec{.interpreted-text role="ref"} for a description of the standard formatting syntax.

The return value must be a string object.

The default implementation by the object{.interpreted-text role="class"} class should be given an empty format_spec string. It delegates to __str__{.interpreted-text role="meth"}.

::: versionchanged 3.4 The __format__ method of object itself raises a TypeError{.interpreted-text role="exc"} if passed any non-empty string. :::

::: versionchanged 3.7 object.__format__(x, '') is now equivalent to str(x) rather than format(str(x), ''). ::: :::::

::: index single: comparisons :::

These are the so-called "rich comparison" methods. The correspondence between operator symbols and method names is as follows: x<y calls x.__lt__(y), x<=y calls x.__le__(y), x==y calls x.__eq__(y), x!=y calls x.__ne__(y), x>y calls x.__gt__(y), and x>=y calls x.__ge__(y).

A rich comparison method may return the singleton NotImplemented{.interpreted-text role="data"} if it does not implement the operation for a given pair of arguments. By convention, False and True 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 if statement), Python will call bool{.interpreted-text role="func"} on the value to determine if the result is true or false.

By default, object implements __eq__{.interpreted-text role="meth"} by using is, returning NotImplemented{.interpreted-text role="data"} in the case of a false comparison: True if x is y else NotImplemented. For __ne__{.interpreted-text role="meth"}, by default it delegates to __eq__{.interpreted-text role="meth"} and inverts the result unless it is !NotImplemented{.interpreted-text role="data"}. There are no other implied relationships among the comparison operators or default implementations; for example, the truth of (x<y or x==y) does not imply x<=y. To automatically generate ordering operations from a single root operation, see functools.total_ordering{.interpreted-text role="func"}.

By default, the object{.interpreted-text role="class"} class provides implementations consistent with expressions-value-comparisons{.interpreted-text role="ref"}: equality compares according to object identity, and order comparisons raise TypeError{.interpreted-text role="exc"}. Each default method may generate these results directly, but may also return NotImplemented{.interpreted-text role="data"}.

See the paragraph on __hash__{.interpreted-text role="meth"} for some important notes on creating hashable{.interpreted-text role="term"} objects which support custom comparison operations and are usable as dictionary keys.

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, __lt__{.interpreted-text role="meth"} and __gt__{.interpreted-text role="meth"} are each other's reflection, __le__{.interpreted-text role="meth"} and __ge__{.interpreted-text role="meth"} are each other's reflection, and __eq__{.interpreted-text role="meth"} and __ne__{.interpreted-text role="meth"} are their own reflection. If the operands are of different types, and the right operand's type is a direct or indirect subclass of the left operand's type, the reflected method of the right operand has priority, otherwise the left operand's method has priority. Virtual subclassing is not considered.

When no appropriate method returns any value other than NotImplemented{.interpreted-text role="data"}, the == and != operators will fall back to is and is not, respectively.

::::::::: method object.__hash__(self)

::: index pair: object; dictionary pair: built-in function; hash :::

Called by built-in function hash{.interpreted-text role="func"} and for operations on members of hashed collections including set{.interpreted-text role="class"}, frozenset{.interpreted-text role="class"}, and dict{.interpreted-text role="class"}. The __hash__() method 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:

def __hash__(self):
    return hash((self.name, self.nick, self.color))

:::: note ::: title Note :::

hash{.interpreted-text role="func"} truncates the value returned from an object's custom __hash__{.interpreted-text role="meth"} method to the size of a Py_ssize_t{.interpreted-text role="c:type"}. This is typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds. If an object's __hash__{.interpreted-text role="meth"} must interoperate on builds of different bit sizes, be sure to check the width on all supported builds. An easy way to do this is with python -c "import sys; print(sys.hash_info.width)". ::::

If a class does not define an __eq__{.interpreted-text role="meth"} method it should not define a __hash__{.interpreted-text role="meth"} operation either; if it defines __eq__{.interpreted-text role="meth"} but not __hash__{.interpreted-text role="meth"}, its instances will not be usable as items in hashable collections. If a class defines mutable objects and implements an __eq__{.interpreted-text role="meth"} method, it should not implement __hash__{.interpreted-text role="meth"}, since the implementation of hashable{.interpreted-text role="term"} collections requires that a key's hash value is immutable (if the object's hash value changes, it will be in the wrong hash bucket).

User-defined classes have __eq__{.interpreted-text role="meth"} and __hash__{.interpreted-text role="meth"} methods by default (inherited from the object{.interpreted-text role="class"} class); with them, all objects compare unequal (except with themselves) and x.__hash__() returns an appropriate value such that x == y implies both that x is y and hash(x) == hash(y).

A class that overrides __eq__{.interpreted-text role="meth"} and does not define __hash__{.interpreted-text role="meth"} will have its __hash__{.interpreted-text role="meth"} implicitly set to None. When the __hash__{.interpreted-text role="meth"} method of a class is None, instances of the class will raise an appropriate TypeError{.interpreted-text role="exc"} when a program attempts to retrieve their hash value, and will also be correctly identified as unhashable when checking isinstance(obj, collections.abc.Hashable).

If a class that overrides __eq__{.interpreted-text role="meth"} needs to retain the implementation of __hash__{.interpreted-text role="meth"} from a parent class, the interpreter must be told this explicitly by setting __hash__ = <ParentClass>.__hash__.

If a class that does not override __eq__{.interpreted-text role="meth"} wishes to suppress hash support, it should include __hash__ = None in the class definition. A class which defines its own __hash__{.interpreted-text role="meth"} that explicitly raises a TypeError{.interpreted-text role="exc"} would be incorrectly identified as hashable by an isinstance(obj, collections.abc.Hashable) call.

:::: note ::: title Note :::

By default, the __hash__{.interpreted-text role="meth"} values of str and bytes objects are "salted" with an unpredictable random value. Although they remain constant within an individual Python process, they are not predictable between repeated invocations of Python.

This is intended to provide protection against a denial-of-service caused by carefully chosen inputs that exploit the worst case performance of a dict insertion, O(n^2^) complexity. See http://ocert.org/advisories/ocert-2011-003.html for details.

Changing hash values affects the iteration order of sets. Python has never made guarantees about this ordering (and it typically varies between 32-bit and 64-bit builds).

See also PYTHONHASHSEED{.interpreted-text role="envvar"}. ::::

::: versionchanged 3.3 Hash randomization is enabled by default. ::: :::::::::

:::: method object.__bool__(self)

::: index single: __len__() (mapping object method) :::

Called to implement truth value testing and the built-in operation bool(); should return False or True. When this method is not defined, ~object.__len__{.interpreted-text role="meth"} is called, if it is defined, and the object is considered true if its result is nonzero. If a class defines neither !__len__{.interpreted-text role="meth"} nor !__bool__{.interpreted-text role="meth"} (which is true of the object{.interpreted-text role="class"} class itself), all its instances are considered true. ::::

Customizing attribute access {#attribute-access}

The following methods can be defined to customize the meaning of attribute access (use of, assignment to, or deletion of x.name) for class instances.

::: method object.__getattr__(self, name)

Called when the default attribute access fails with an AttributeError{.interpreted-text role="exc"} (either __getattribute__{.interpreted-text role="meth"} raises an AttributeError{.interpreted-text role="exc"} because name is not an instance attribute or an attribute in the class tree for self; or __get__{.interpreted-text role="meth"} of a name property raises AttributeError{.interpreted-text role="exc"}). This method should either return the (computed) attribute value or raise an AttributeError{.interpreted-text role="exc"} exception. The object{.interpreted-text role="class"} class itself does not provide this method.

Note that if the attribute is found through the normal mechanism, __getattr__{.interpreted-text role="meth"} is not called. (This is an intentional asymmetry between __getattr__{.interpreted-text role="meth"} and __setattr__{.interpreted-text role="meth"}.) This is done both for efficiency reasons and because otherwise __getattr__{.interpreted-text role="meth"} would have no way to access other attributes of the instance. Note that at least for instance variables, you can take total control by not inserting any values in the instance attribute dictionary (but instead inserting them in another object). See the __getattribute__{.interpreted-text role="meth"} method below for a way to actually get total control over attribute access. :::

:::::: method object.__getattribute__(self, name)

Called unconditionally to implement attribute accesses for instances of the class. If the class also defines __getattr__{.interpreted-text role="meth"}, the latter will not be called unless __getattribute__{.interpreted-text role="meth"} either calls it explicitly or raises an AttributeError{.interpreted-text role="exc"}. This method should return the (computed) attribute value or raise an AttributeError{.interpreted-text role="exc"} 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, object.__getattribute__(self, name).

:::: note ::: title Note :::

This method may still be bypassed when looking up special methods as the result of implicit invocation via language syntax or built-in functions <builtin-functions>{.interpreted-text role="ref"}. See special-lookup{.interpreted-text role="ref"}. ::::

::: audit-event object.__getattr__ obj,name object.__getattribute__

For certain sensitive attribute accesses, raises an auditing event <auditing>{.interpreted-text role="ref"} object.__getattr__ with arguments obj and name. ::: ::::::

:::: method object.__setattr__(self, name, value)

Called when an attribute assignment is attempted. This is called instead of the normal mechanism (i.e. store the value in the instance dictionary). name is the attribute name, value is the value to be assigned to it.

If __setattr__{.interpreted-text role="meth"} wants to assign to an instance attribute, it should call the base class method with the same name, for example, object.__setattr__(self, name, value).

::: audit-event object.__setattr__ obj,name,value object.__setattr__

For certain sensitive attribute assignments, raises an auditing event <auditing>{.interpreted-text role="ref"} object.__setattr__ with arguments obj, name, value. ::: ::::

:::: method object.__delattr__(self, name)

Like __setattr__{.interpreted-text role="meth"} but for attribute deletion instead of assignment. This should only be implemented if del obj.name is meaningful for the object.

::: audit-event object.__delattr__ obj,name object.__delattr__

For certain sensitive attribute deletions, raises an auditing event <auditing>{.interpreted-text role="ref"} object.__delattr__ with arguments obj and name. ::: ::::

::: method object.__dir__(self)

Called when dir{.interpreted-text role="func"} is called on the object. An iterable must be returned. dir{.interpreted-text role="func"} converts the returned iterable to a list and sorts it. :::

Customizing module attribute access

::: index single: __getattr__ (module attribute) single: __dir__ (module attribute) single: __class__ (module attribute) :::

::: method module.__getattr__ module.__dir__ :::

Special names __getattr__ and __dir__ can be also used to customize access to module attributes. The __getattr__ function at the module level should accept one argument which is the name of an attribute and return the computed value or raise an AttributeError{.interpreted-text role="exc"}. If an attribute is not found on a module object through the normal lookup, i.e. object.__getattribute__{.interpreted-text role="meth"}, then __getattr__ is searched in the module __dict__ before raising an AttributeError{.interpreted-text role="exc"}. If found, it is called with the attribute name and the result is returned.

The __dir__ function should accept no arguments, and return an iterable of strings that represents the names accessible on module. If present, this function overrides the standard dir{.interpreted-text role="func"} search on a module.

::: attribute module.__class__ :::

For a more fine grained customization of the module behavior (setting attributes, properties, etc.), one can set the __class__ attribute of a module object to a subclass of types.ModuleType{.interpreted-text role="class"}. For example:

import sys
from types import ModuleType

class VerboseModule(ModuleType):
    def __repr__(self):
        return f'Verbose {self.__name__}'

    def __setattr__(self, attr, value):
        print(f'Setting {attr}...')
        super().__setattr__(attr, value)

sys.modules[__name__].__class__ = VerboseModule

:::: note ::: title Note :::

Defining module __getattr__ and setting module __class__ only affect lookups made using the attribute access syntax -- directly accessing the module globals (whether by code within the module, or via a reference to the module's globals dictionary) is unaffected. ::::

::: versionchanged 3.5 __class__ module attribute is now writable. :::

::: versionadded 3.7 __getattr__ and __dir__ module attributes. :::

::: seealso

562{.interpreted-text role="pep"} - Module __getattr__ and __dir__

: Describes the __getattr__ and __dir__ functions on modules. :::

Implementing Descriptors {#descriptors}

The following methods only apply when an instance of the class containing the method (a so-called descriptor class) appears in an owner 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' ~object.__dict__{.interpreted-text role="attr"}. The object{.interpreted-text role="class"} class itself does not implement any of these protocols.

::: method object.__get__(self, instance, owner=None)

Called to get the attribute of the owner class (class attribute access) or of an instance of that class (instance attribute access). The optional owner argument is the owner class, while instance is the instance that the attribute was accessed through, or None when the attribute is accessed through the owner.

This method should return the computed attribute value or raise an AttributeError{.interpreted-text role="exc"} exception.

PEP 252 specifies that __get__{.interpreted-text role="meth"} is callable with one or two arguments. Python's own built-in descriptors support this specification; however, it is likely that some third-party tools have descriptors that require both arguments. Python's own __getattribute__{.interpreted-text role="meth"} implementation always passes in both arguments whether they are required or not. :::

::: method object.__set__(self, instance, value)

Called to set the attribute on an instance instance of the owner class to a new value, value.

Note, adding __set__{.interpreted-text role="meth"} or __delete__{.interpreted-text role="meth"} changes the kind of descriptor to a "data descriptor". See descriptor-invocation{.interpreted-text role="ref"} for more details. :::

::: method object.__delete__(self, instance)

Called to delete the attribute on an instance instance of the owner class. :::

Instances of descriptors may also have the !__objclass__{.interpreted-text role="attr"} attribute present:

::: attribute object.__objclass__

The attribute !__objclass__{.interpreted-text role="attr"} is interpreted by the inspect{.interpreted-text role="mod"} module as specifying the class where this object was defined (setting this appropriately can assist in runtime introspection of dynamic class attributes). For callables, it may indicate that an instance of the given type (or a subclass) is expected or required as the first positional argument (for example, CPython sets this attribute for unbound methods that are implemented in C). :::

Invoking Descriptors {#descriptor-invocation}

In general, a descriptor is an object attribute with "binding behavior", one whose attribute access has been overridden by methods in the descriptor protocol: ~object.__get__{.interpreted-text role="meth"}, ~object.__set__{.interpreted-text role="meth"}, and ~object.__delete__{.interpreted-text role="meth"}. If any of those methods are defined for an object, it is said to be a descriptor.

The default behavior for attribute access is to get, set, or delete the attribute from an object's dictionary. For instance, a.x has a lookup chain starting with a.__dict__['x'], then type(a).__dict__['x'], and continuing through the base classes of type(a) excluding metaclasses.

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.

The starting point for descriptor invocation is a binding, a.x. How the arguments are assembled depends on a:

Direct Call

: The simplest and least common call is when user code directly invokes a descriptor method: x.__get__(a).

Instance Binding

: If binding to an object instance, a.x is transformed into the call: type(a).__dict__['x'].__get__(a, type(a)).

Class Binding

: If binding to a class, A.x is transformed into the call: A.__dict__['x'].__get__(None, A).

Super Binding

: A dotted lookup such as super(A, a).x searches a.__class__.__mro__ for a base class B following A and then returns B.__dict__['x'].__get__(a, A). If not a descriptor, x is returned unchanged.

::: {.testcode hide=""}

class Desc:

:

def \_\_get\_\_(\*args):

:   return args

class B:

x = Desc()

class A(B):

x = 999

def m(self):

: 'Demonstrate these two descriptor invocations are equivalent' result1 = super(A, self).x result2 = B.__dict__['x'].__get__(self, A) return result1 == result2 :::

::: {.doctest hide=""} >>> a = A() >>> a.__class__.__mro__.index(B) > a.__class__.__mro__.index(A) True >>> super(A, a).x == B.__dict__['x'].__get__(a, A) True >>> a.m() True :::

For instance bindings, the precedence of descriptor invocation depends on which descriptor methods are defined. A descriptor can define any combination of ~object.__get__{.interpreted-text role="meth"}, ~object.__set__{.interpreted-text role="meth"} and ~object.__delete__{.interpreted-text role="meth"}. If it does not define !__get__{.interpreted-text role="meth"}, 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 !__set__{.interpreted-text role="meth"} and/or !__delete__{.interpreted-text role="meth"}, it is a data descriptor; if it defines neither, it is a non-data descriptor. Normally, data descriptors define both !__get__{.interpreted-text role="meth"} and !__set__{.interpreted-text role="meth"}, while non-data descriptors have just the !__get__{.interpreted-text role="meth"} method. Data descriptors with !__get__{.interpreted-text role="meth"} and !__set__{.interpreted-text role="meth"} (and/or !__delete__{.interpreted-text role="meth"}) defined always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by instances.

Python methods (including those decorated with staticmethod{.interpreted-text role="deco"} and classmethod{.interpreted-text role="deco"}) 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.

The property{.interpreted-text role="func"} function is implemented as a data descriptor. Accordingly, instances cannot override the behavior of a property.

__slots__ {#slots}

__slots__ allow us to explicitly declare data members (like properties) and deny the creation of ~object.__dict__{.interpreted-text role="attr"} and __weakref__ (unless explicitly declared in __slots__ or available in a parent.)

The space saved over using ~object.__dict__{.interpreted-text role="attr"} can be significant. Attribute lookup speed can be significantly improved as well.

::: data object.__slots__

This class variable can be assigned a string, iterable, or sequence of strings with variable names used by instances. __slots__ reserves space for the declared variables and prevents the automatic creation of ~object.__dict__{.interpreted-text role="attr"} and __weakref__ for each instance. :::

::: {#datamodel-note-slots} Notes on using __slots__: :::

• When inheriting from a class without __slots__, the ~object.__dict__{.interpreted-text role="attr"} and __weakref__ attribute of the instances will always be accessible.
• Without a ~object.__dict__{.interpreted-text role="attr"} variable, instances cannot be assigned new variables not listed in the __slots__ definition. Attempts to assign to an unlisted variable name raises AttributeError{.interpreted-text role="exc"}. If dynamic assignment of new variables is desired, then add '__dict__' to the sequence of strings in the __slots__ declaration.
• Without a __weakref__ variable for each instance, classes defining __slots__ do not support weak references <weakref>{.interpreted-text role="mod"} to its instances. If weak reference support is needed, then add '__weakref__' to the sequence of strings in the __slots__ declaration.
• __slots__ are implemented at the class level by creating descriptors <descriptors>{.interpreted-text role="ref"} for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by __slots__; otherwise, the class attribute would overwrite the descriptor assignment.
• The action of a __slots__ declaration is not limited to the class where it is defined. __slots__ declared in parents are available in child classes. However, instances of a child subclass will get a ~object.__dict__{.interpreted-text role="attr"} and __weakref__ unless the subclass also defines __slots__ (which should only contain names of any additional slots).
• 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.
• TypeError{.interpreted-text role="exc"} will be raised if __slots__ other than __dict__ and __weakref__ are defined for a class derived from a "variable-length" built-in type <PyTypeObject.tp_itemsize>{.interpreted-text role="c:member"} such as int{.interpreted-text role="class"}, bytes{.interpreted-text role="class"}, and type{.interpreted-text role="class"}, except tuple{.interpreted-text role="class"}.
• Any non-string iterable{.interpreted-text role="term"} may be assigned to __slots__.
• If a dictionary <dict>{.interpreted-text role="class"} is used to assign __slots__, the dictionary keys will be used as the slot names. The values of the dictionary can be used to provide per-attribute docstrings that will be recognised by inspect.getdoc{.interpreted-text role="func"} and displayed in the output of help{.interpreted-text role="func"}.
• ~object.__class__{.interpreted-text role="attr"} assignment works only if both classes have the same __slots__.
• Multiple inheritance <multiple-inheritance>{.interpreted-text role="ref"} with multiple slotted parent classes can be used, but only one parent is allowed to have attributes created by slots (the other bases must have empty slot layouts) - violations raise TypeError{.interpreted-text role="exc"}.
• If an iterator{.interpreted-text role="term"} is used for __slots__ then a descriptor{.interpreted-text role="term"} is created for each of the iterator's values. However, the __slots__ attribute will be an empty iterator.

::: versionchanged 3.15 Allowed defining the __dict__ and __weakref__ __slots__ for any class. Allowed defining any __slots__ for a class derived from tuple{.interpreted-text role="class"}. :::

Customizing class creation {#class-customization}

Whenever a class inherits from another class, ~object.__init_subclass__{.interpreted-text role="meth"} is called on the parent class. This way, it is possible to write classes which change the behavior of subclasses. This is closely related to class decorators, but where class decorators only affect the specific class they're applied to, __init_subclass__ solely applies to future subclasses of the class defining the method.

:::::: classmethod object.__init_subclass__(cls)

This method is called whenever the containing class is subclassed. cls is then the new subclass. If defined as a normal instance method, this method is implicitly converted to a class method.

Keyword arguments which are given to a new class are passed to the parent class's __init_subclass__. For compatibility with other classes using __init_subclass__, one should take out the needed keyword arguments and pass the others over to the base class, as in:

class Philosopher:
    def __init_subclass__(cls, /, default_name, **kwargs):
        super().__init_subclass__(**kwargs)
        cls.default_name = default_name

class AustralianPhilosopher(Philosopher, default_name="Bruce"):
    pass

The default implementation object.__init_subclass__ does nothing, but raises an error if it is called with any arguments.

:::: note ::: title Note :::

The metaclass hint metaclass is consumed by the rest of the type machinery, and is never passed to __init_subclass__ implementations. The actual metaclass (rather than the explicit hint) can be accessed as type(cls). ::::

::: versionadded 3.6 ::: ::::::

When a class is created, !type.__new__{.interpreted-text role="meth"} scans the class variables and makes callbacks to those with a ~object.__set_name__{.interpreted-text role="meth"} hook.

:::: method object.__set_name__(self, owner, name)

Automatically called at the time the owning class owner is created. The object has been assigned to name in that class:

class A:
    x = C()  # Automatically calls: x.__set_name__(A, 'x')

If the class variable is assigned after the class is created, __set_name__{.interpreted-text role="meth"} will not be called automatically. If needed, __set_name__{.interpreted-text role="meth"} can be called directly:

class A:
   pass

c = C()
A.x = c                  # The hook is not called
c.__set_name__(A, 'x')   # Manually invoke the hook

See class-object-creation{.interpreted-text role="ref"} for more details.

::: versionadded 3.6 ::: ::::

Metaclasses

::: index single: metaclass pair: built-in function; type single: = (equals); class definition :::

By default, classes are constructed using type{.interpreted-text role="func"}. The class body is executed in a new namespace and the class name is bound locally to the result of type(name, bases, namespace).

The class creation process can be customized by passing the metaclass keyword argument in the class definition line, or by inheriting from an existing class that included such an argument. In the following example, both MyClass and MySubclass are instances of Meta:

class Meta(type):
    pass

class MyClass(metaclass=Meta):
    pass

class MySubclass(MyClass):
    pass

Any other keyword arguments that are specified in the class definition are passed through to all metaclass operations described below.

When a class definition is executed, the following steps occur:

• MRO entries are resolved;
• the appropriate metaclass is determined;
• the class namespace is prepared;
• the class body is executed;
• the class object is created.

Resolving MRO entries

::: method object.__mro_entries__(self, bases)

If a base that appears in a class definition is not an instance of type{.interpreted-text role="class"}, then an !__mro_entries__{.interpreted-text role="meth"} method is searched on the base. If an !__mro_entries__{.interpreted-text role="meth"} method is found, the base is substituted with the result of a call to !__mro_entries__{.interpreted-text role="meth"} when creating the class. The method is called with the original bases tuple passed to the bases parameter, and must return a tuple of classes that will be used instead of the base. The returned tuple may be empty: in these cases, the original base is ignored. :::

::: seealso

types.resolve_bases{.interpreted-text role="func"}

: Dynamically resolve bases that are not instances of type{.interpreted-text role="class"}.

types.get_original_bases{.interpreted-text role="func"}

: Retrieve a class's "original bases" prior to modifications by ~object.__mro_entries__{.interpreted-text role="meth"}.

560{.interpreted-text role="pep"}

: Core support for typing module and generic types. :::

Determining the appropriate metaclass {#metaclass-determination}

::: index single: metaclass hint :::

The appropriate metaclass for a class definition is determined as follows:

• if no bases and no explicit metaclass are given, then type{.interpreted-text role="func"} is used;
• if an explicit metaclass is given and it is not an instance of type{.interpreted-text role="func"}, then it is used directly as the metaclass;
• if an instance of type{.interpreted-text role="func"} is given as the explicit metaclass, or bases are defined, then the most derived metaclass is used.

The most derived metaclass is selected from the explicitly specified metaclass (if any) and the metaclasses (i.e. type(cls)) of all specified base classes. The most derived metaclass is one which is a subtype of all of these candidate metaclasses. If none of the candidate metaclasses meets that criterion, then the class definition will fail with TypeError.

Preparing the class namespace {#prepare}

::: index single: __prepare__ (metaclass method) :::

Once the appropriate metaclass has been identified, then the class namespace is prepared. If the metaclass has a __prepare__ attribute, it is called as namespace = metaclass.__prepare__(name, bases, **kwds) (where the additional keyword arguments, if any, come from the class definition). The __prepare__ method should be implemented as a classmethod <classmethod>{.interpreted-text role="func"}. The namespace returned by __prepare__ is passed in to __new__, but when the final class object is created the namespace is copied into a new dict.

If the metaclass has no __prepare__ attribute, then the class namespace is initialised as an empty ordered mapping.

::: seealso

3115{.interpreted-text role="pep"} - Metaclasses in Python 3000

: Introduced the __prepare__ namespace hook :::

Executing the class body

::: index single: class; body :::

The class body is executed (approximately) as exec(body, globals(), namespace). The key difference from a normal call to exec{.interpreted-text role="func"} is that lexical scoping allows the class body (including any methods) to reference names from the current and outer scopes when the class definition occurs inside a function.

However, even when the class definition occurs inside the function, methods defined inside the class still cannot see names defined at the class scope. Class variables must be accessed through the first parameter of instance or class methods, or through the implicit lexically scoped __class__ reference described in the next section.

Creating the class object {#class-object-creation}

::: index single: __class__ (method cell) single: __classcell__ (class namespace entry) :::

Once the class namespace has been populated by executing the class body, the class object is created by calling metaclass(name, bases, namespace, **kwds) (the additional keywords passed here are the same as those passed to __prepare__).

This class object is the one that will be referenced by the zero-argument form of super{.interpreted-text role="func"}. __class__ is an implicit closure reference created by the compiler if any methods in a class body refer to either __class__ or super. This allows the zero argument form of super{.interpreted-text role="func"} to correctly identify the class being defined based on lexical scoping, while the class or instance that was used to make the current call is identified based on the first argument passed to the method.

::: impl-detail In CPython 3.6 and later, the __class__ cell is passed to the metaclass as a __classcell__ entry in the class namespace. If present, this must be propagated up to the type.__new__ call in order for the class to be initialised correctly. Failing to do so will result in a RuntimeError{.interpreted-text role="exc"} in Python 3.8. :::

When using the default metaclass type{.interpreted-text role="class"}, or any metaclass that ultimately calls type.__new__, the following additional customization steps are invoked after creating the class object:

1. The type.__new__ method collects all of the attributes in the class namespace that define a ~object.__set_name__{.interpreted-text role="meth"} method;
2. Those __set_name__ methods are called with the class being defined and the assigned name of that particular attribute;
3. The ~object.__init_subclass__{.interpreted-text role="meth"} hook is called on the immediate parent of the new class in its method resolution order.

After the class object is created, it is passed to the class decorators included in the class definition (if any) and the resulting object is bound in the local namespace as the defined class.

When a new class is created by type.__new__, the object provided as the namespace parameter is copied to a new ordered mapping and the original object is discarded. The new copy is wrapped in a read-only proxy, which becomes the ~type.__dict__{.interpreted-text role="attr"} attribute of the class object.

::: seealso

3135{.interpreted-text role="pep"} - New super

: Describes the implicit __class__ closure reference :::

Uses for metaclasses

The potential uses for metaclasses are boundless. Some ideas that have been explored include enum, logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization.

Customizing instance and subclass checks

The following methods are used to override the default behavior of the isinstance{.interpreted-text role="func"} and issubclass{.interpreted-text role="func"} built-in functions.

In particular, the metaclass abc.ABCMeta{.interpreted-text role="class"} 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.

::: method type.__instancecheck__(self, instance)

Return true if instance should be considered a (direct or indirect) instance of class. If defined, called to implement isinstance(instance, class). :::

::: method type.__subclasscheck__(self, subclass)

Return true if subclass should be considered a (direct or indirect) subclass of class. If defined, called to implement issubclass(subclass, class). :::

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.

::: seealso

3119{.interpreted-text role="pep"} - Introducing Abstract Base Classes

: Includes the specification for customizing isinstance{.interpreted-text role="func"} and issubclass{.interpreted-text role="func"} behavior through ~type.__instancecheck__{.interpreted-text role="meth"} and ~type.__subclasscheck__{.interpreted-text role="meth"}, with motivation for this functionality in the context of adding Abstract Base Classes (see the abc{.interpreted-text role="mod"} module) to the language. :::

Emulating generic types

When using type annotations<annotation>{.interpreted-text role="term"}, it is often useful to parameterize a generic type{.interpreted-text role="term"} using Python's square-brackets notation. For example, the annotation list[int] might be used to signify a list{.interpreted-text role="class"} in which all the elements are of type int{.interpreted-text role="class"}.

::: seealso

484{.interpreted-text role="pep"} - Type Hints

: Introducing Python's framework for type annotations

Generic Alias Types<types-genericalias>{.interpreted-text role="ref"}

: Documentation for objects representing parameterized generic classes

Generics{.interpreted-text role="ref"}, user-defined generics<user-defined-generics>{.interpreted-text role="ref"} and typing.Generic{.interpreted-text role="class"}

: Documentation on how to implement generic classes that can be parameterized at runtime and understood by static type-checkers. :::

A class can generally only be parameterized if it defines the special class method __class_getitem__().

::: classmethod object.__class_getitem__(cls, key)

Return an object representing the specialization of a generic class by type arguments found in key.

When defined on a class, __class_getitem__() is automatically a class method. As such, there is no need for it to be decorated with classmethod{.interpreted-text role="deco"} when it is defined. :::

The purpose of __class_getitem__

The purpose of ~object.__class_getitem__{.interpreted-text role="meth"} is to allow runtime parameterization of standard-library generic classes in order to more easily apply type hints<type hint>{.interpreted-text role="term"} to these classes.

To implement custom generic classes that can be parameterized at runtime and understood by static type-checkers, users should either inherit from a standard library class that already implements ~object.__class_getitem__{.interpreted-text role="meth"}, or inherit from typing.Generic{.interpreted-text role="class"}, which has its own implementation of __class_getitem__().

Custom implementations of ~object.__class_getitem__{.interpreted-text role="meth"} on classes defined outside of the standard library may not be understood by third-party type-checkers such as mypy. Using __class_getitem__() on any class for purposes other than type hinting is discouraged.

__class_getitem__ versus __getitem__ {#classgetitem-versus-getitem}

Usually, the subscription<subscriptions>{.interpreted-text role="ref"} of an object using square brackets will call the ~object.__getitem__{.interpreted-text role="meth"} instance method defined on the object's class. However, if the object being subscribed is itself a class, the class method ~object.__class_getitem__{.interpreted-text role="meth"} may be called instead. __class_getitem__() should return a GenericAlias<types-genericalias>{.interpreted-text role="ref"} object if it is properly defined.

Presented with the expression{.interpreted-text role="term"} obj[x], the Python interpreter follows something like the following process to decide whether ~object.__getitem__{.interpreted-text role="meth"} or ~object.__class_getitem__{.interpreted-text role="meth"} should be called:

from inspect import isclass

def subscribe(obj, x):
    """Return the result of the expression 'obj[x]'"""

    class_of_obj = type(obj)

    # If the class of obj defines __getitem__,
    # call class_of_obj.__getitem__(obj, x)
    if hasattr(class_of_obj, '__getitem__'):
        return class_of_obj.__getitem__(obj, x)

    # Else, if obj is a class and defines __class_getitem__,
    # call obj.__class_getitem__(x)
    elif isclass(obj) and hasattr(obj, '__class_getitem__'):
        return obj.__class_getitem__(x)

    # Else, raise an exception
    else:
        raise TypeError(
            f"'{class_of_obj.__name__}' object is not subscriptable"
        )

In Python, all classes are themselves instances of other classes. The class of a class is known as that class's metaclass{.interpreted-text role="term"}, and most classes have the type{.interpreted-text role="class"} class as their metaclass. type{.interpreted-text role="class"} does not define ~object.__getitem__{.interpreted-text role="meth"}, meaning that expressions such as list[int], dict[str, float] and tuple[str, bytes] all result in ~object.__class_getitem__{.interpreted-text role="meth"} being called:

>>> # list has class "type" as its metaclass, like most classes:
>>> type(list)
<class 'type'>
>>> type(dict) == type(list) == type(tuple) == type(str) == type(bytes)
True
>>> # "list[int]" calls "list.__class_getitem__(int)"
>>> list[int]
list[int]
>>> # list.__class_getitem__ returns a GenericAlias object:
>>> type(list[int])
<class 'types.GenericAlias'>

However, if a class has a custom metaclass that defines ~object.__getitem__{.interpreted-text role="meth"}, subscribing the class may result in different behaviour. An example of this can be found in the enum{.interpreted-text role="mod"} module:

>>> from enum import Enum
>>> class Menu(Enum):
...     """A breakfast menu"""
...     SPAM = 'spam'
...     BACON = 'bacon'
...
>>> # Enum classes have a custom metaclass:
>>> type(Menu)
<class 'enum.EnumMeta'>
>>> # EnumMeta defines __getitem__,
>>> # so __class_getitem__ is not called,
>>> # and the result is not a GenericAlias object:
>>> Menu['SPAM']
<Menu.SPAM: 'spam'>
>>> type(Menu['SPAM'])
<enum 'Menu'>

::: seealso 560{.interpreted-text role="pep"} - Core Support for typing module and generic types Introducing ~object.__class_getitem__{.interpreted-text role="meth"}, and outlining when a subscription<subscriptions>{.interpreted-text role="ref"} results in __class_getitem__() being called instead of ~object.__getitem__{.interpreted-text role="meth"} :::

Emulating callable objects {#callable-types}

:::: method object.__call__(self[, args...])

::: index pair: call; instance :::

Called when the instance is "called" as a function; if this method is defined, x(arg1, arg2, ...) roughly translates to type(x).__call__(x, arg1, ...). The object{.interpreted-text role="class"} class itself does not provide this method. ::::

Emulating container types {#sequence-types}

The following methods can be defined to implement container objects. None of them are provided by the object{.interpreted-text role="class"} class itself. Containers usually are sequences <sequence>{.interpreted-text role="term"} (such as lists <list>{.interpreted-text role="class"} or tuples <tuple>{.interpreted-text role="class"}) or mappings <mapping>{.interpreted-text role="term"} (like dictionaries <dictionary>{.interpreted-text role="term"}), 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 k for which 0 <= k < N where N is the length of the sequence, or slice{.interpreted-text role="class"} objects, which define a range of items. It is also recommended that mappings provide the methods !keys{.interpreted-text role="meth"}, !values{.interpreted-text role="meth"}, !items{.interpreted-text role="meth"}, !get{.interpreted-text role="meth"}, !clear{.interpreted-text role="meth"}, !setdefault{.interpreted-text role="meth"}, !pop{.interpreted-text role="meth"}, !popitem{.interpreted-text role="meth"}, !copy{.interpreted-text role="meth"}, and !update{.interpreted-text role="meth"} behaving similar to those for Python's standard dictionary <dict>{.interpreted-text role="class"} objects. The collections.abc{.interpreted-text role="mod"} module provides a ~collections.abc.MutableMapping{.interpreted-text role="class"} abstract base class{.interpreted-text role="term"} to help create those methods from a base set of ~object.__getitem__{.interpreted-text role="meth"}, ~object.__setitem__{.interpreted-text role="meth"}, ~object.__delitem__{.interpreted-text role="meth"}, and !keys{.interpreted-text role="meth"}.

Mutable sequences should provide methods ~sequence.append{.interpreted-text role="meth"}, ~sequence.clear{.interpreted-text role="meth"}, ~sequence.count{.interpreted-text role="meth"}, ~sequence.extend{.interpreted-text role="meth"}, ~sequence.index{.interpreted-text role="meth"}, ~sequence.insert{.interpreted-text role="meth"}, ~sequence.pop{.interpreted-text role="meth"}, ~sequence.remove{.interpreted-text role="meth"}, and ~sequence.reverse{.interpreted-text role="meth"}, like Python standard list{.interpreted-text role="class"} objects. Finally, sequence types should implement addition (meaning concatenation) and multiplication (meaning repetition) by defining the methods ~object.__add__{.interpreted-text role="meth"}, ~object.__radd__{.interpreted-text role="meth"}, ~object.__iadd__{.interpreted-text role="meth"}, ~object.__mul__{.interpreted-text role="meth"}, ~object.__rmul__{.interpreted-text role="meth"} and ~object.__imul__{.interpreted-text role="meth"} described below; they should not define other numerical operators.

It is recommended that both mappings and sequences implement the ~object.__contains__{.interpreted-text role="meth"} method to allow efficient use of the in operator; for mappings, in should search the mapping's keys; for sequences, it should search through the values. It is further recommended that both mappings and sequences implement the ~object.__iter__{.interpreted-text role="meth"} method to allow efficient iteration through the container; for mappings, !__iter__{.interpreted-text role="meth"} should iterate through the object's keys; for sequences, it should iterate through the values.

::::: method object.__len__(self)

::: index pair: built-in function; len single: __bool__() (object method) :::

Called to implement the built-in function len{.interpreted-text role="func"}. Should return the length of the object, an integer >= 0. Also, an object that doesn't define a ~object.__bool__{.interpreted-text role="meth"} method and whose !__len__{.interpreted-text role="meth"} method returns zero is considered to be false in a Boolean context.

::: impl-detail In CPython, the length is required to be at most sys.maxsize{.interpreted-text role="data"}. If the length is larger than !sys.maxsize{.interpreted-text role="data"} some features (such as len{.interpreted-text role="func"}) may raise OverflowError{.interpreted-text role="exc"}. To prevent raising !OverflowError{.interpreted-text role="exc"} by truth value testing, an object must define a ~object.__bool__{.interpreted-text role="meth"} method. ::: :::::

:::: method object.__length_hint__(self)

Called to implement operator.length_hint{.interpreted-text role="func"}. Should return an estimated length for the object (which may be greater or less than the actual length). The length must be an integer >= 0. The return value may also be NotImplemented{.interpreted-text role="data"}, which is treated the same as if the __length_hint__ method didn't exist at all. This method is purely an optimization and is never required for correctness.

::: versionadded 3.4 ::: ::::

::: index pair: object; slice :::

:::: note ::: title Note :::

Slicing is done exclusively with the following three methods. A call like :

a[1:2] = b

is translated to :

a[slice(1, 2, None)] = b

and so forth. Missing slice items are always filled in with None. ::::

::::::: method object.__getitem__(self, subscript)

Called to implement subscription, that is, self[subscript]. See subscriptions{.interpreted-text role="ref"} for details on the syntax.

There are two types of built-in objects that support subscription via !__getitem__{.interpreted-text role="meth"}:

• sequences, where subscript (also called index{.interpreted-text role="term"}) should be an integer or a slice{.interpreted-text role="class"} object. See the sequence documentation <datamodel-sequences>{.interpreted-text role="ref"} for the expected behavior, including handling slice{.interpreted-text role="class"} objects and negative indices.
• mappings, where subscript is also called the key{.interpreted-text role="term"}. See mapping documentation <datamodel-mappings>{.interpreted-text role="ref"} for the expected behavior.

If subscript is of an inappropriate type, !__getitem__{.interpreted-text role="meth"} should raise TypeError{.interpreted-text role="exc"}. If subscript has an inappropriate value, !__getitem__{.interpreted-text role="meth"} should raise an LookupError{.interpreted-text role="exc"} or one of its subclasses (IndexError{.interpreted-text role="exc"} for sequences; KeyError{.interpreted-text role="exc"} for mappings).

:::: note ::: title Note :::

The sequence iteration protocol (used, for example, in for{.interpreted-text role="keyword"} loops), expects that an IndexError{.interpreted-text role="exc"} will be raised for illegal indexes to allow proper detection of the end of a sequence. ::::

:::: note ::: title Note :::

When subscripting <subscriptions>{.interpreted-text role="ref"} a class, the special class method ~object.__class_getitem__{.interpreted-text role="meth"} may be called instead of !__getitem__{.interpreted-text role="meth"}. See classgetitem-versus-getitem{.interpreted-text role="ref"} for more details. :::: :::::::

::: method object.__setitem__(self, key, value)

Called to implement assignment to self[key]. Same note as for __getitem__{.interpreted-text role="meth"}. 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 key values as for the __getitem__{.interpreted-text role="meth"} method. :::

::: method object.__delitem__(self, key)

Called to implement deletion of self[key]. Same note as for __getitem__{.interpreted-text role="meth"}. 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 key values as for the __getitem__{.interpreted-text role="meth"} method. :::

::: method object.__missing__(self, key)

Called by dict{.interpreted-text role="class"}.__getitem__{.interpreted-text role="meth"} to implement self[key] for dict subclasses when key is not in the dictionary. :::

::: method object.__iter__(self)

This method is called when an iterator{.interpreted-text role="term"} 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. :::

::: method object.__reversed__(self)

Called (if present) by the reversed{.interpreted-text role="func"} 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.

If the __reversed__{.interpreted-text role="meth"} method is not provided, the reversed{.interpreted-text role="func"} built-in will fall back to using the sequence protocol (__len__{.interpreted-text role="meth"} and __getitem__{.interpreted-text role="meth"}). Objects that support the sequence protocol should only provide __reversed__{.interpreted-text role="meth"} if they can provide an implementation that is more efficient than the one provided by reversed{.interpreted-text role="func"}. :::

The membership test operators (in{.interpreted-text role="keyword"} and not in{.interpreted-text role="keyword"}) are normally implemented as an iteration through a container. However, container objects can supply the following special method with a more efficient implementation, which also does not require the object be iterable.

::: method object.__contains__(self, item)

Called to implement membership test operators. Should return true if item is in self, false otherwise. For mapping objects, this should consider the keys of the mapping rather than the values or the key-item pairs.

For objects that don't define __contains__{.interpreted-text role="meth"}, the membership test first tries iteration via __iter__{.interpreted-text role="meth"}, then the old sequence iteration protocol via __getitem__{.interpreted-text role="meth"}, see this section in the language reference <membership-test-details>{.interpreted-text role="ref"}. :::

Emulating numeric types {#numeric-types}

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.

:::: method object.__add__(self, other) object.__sub__(self, other) object.__mul__(self, other) object.__matmul__(self, other) object.__truediv__(self, other) object.__floordiv__(self, other) object.__mod__(self, other) object.__divmod__(self, other) object.__pow__(self, other[, modulo]) object.__lshift__(self, other) object.__rshift__(self, other) object.__and__(self, other) object.__xor__(self, other) object.__or__(self, other)

::: index pair: built-in function; divmod pair: built-in function; pow pair: built-in function; pow :::

These methods are called to implement the binary arithmetic operations (+, -, *, @, /, //, %, divmod{.interpreted-text role="func"}, pow{.interpreted-text role="func"}, **, <<, >>, &, ^, |). For instance, to evaluate the expression x + y, where x is an instance of a class that has an __add__{.interpreted-text role="meth"} method, type(x).__add__(x, y) is called. The __divmod__{.interpreted-text role="meth"} method should be the equivalent to using __floordiv__{.interpreted-text role="meth"} and __mod__{.interpreted-text role="meth"}; it should not be related to __truediv__{.interpreted-text role="meth"}. Note that __pow__{.interpreted-text role="meth"} should be defined to accept an optional third argument if the three-argument version of the built-in pow{.interpreted-text role="func"} function is to be supported.

If one of those methods does not support the operation with the supplied arguments, it should return NotImplemented{.interpreted-text role="data"}. ::::

::::::: method object.__radd__(self, other) object.__rsub__(self, other) object.__rmul__(self, other) object.__rmatmul__(self, other) object.__rtruediv__(self, other) object.__rfloordiv__(self, other) object.__rmod__(self, other) object.__rdivmod__(self, other) object.__rpow__(self, other[, modulo]) object.__rlshift__(self, other) object.__rrshift__(self, other) object.__rand__(self, other) object.__rxor__(self, other) object.__ror__(self, other)

::: index pair: built-in function; divmod pair: built-in function; pow :::

These methods are called to implement the binary arithmetic operations (+, -, *, @, /, //, %, divmod{.interpreted-text role="func"}, pow{.interpreted-text role="func"}, **, <<, >>, &, ^, |) with reflected (swapped) operands. These functions are only called if the operands are of different types, when the left operand does not support the corresponding operation[^3], or the right operand's class is derived from the left operand's class.[^4] For instance, to evaluate the expression x - y, where y is an instance of a class that has an __rsub__{.interpreted-text role="meth"} method, type(y).__rsub__(y, x) is called if type(x).__sub__(x, y) returns NotImplemented{.interpreted-text role="data"} or type(y) is a subclass of type(x).[^5]

Note that __rpow__{.interpreted-text role="meth"} should be defined to accept an optional third argument if the three-argument version of the built-in pow{.interpreted-text role="func"} function is to be supported.

::: versionchanged 3.14

Three-argument pow{.interpreted-text role="func"} now try calling ~object.__rpow__{.interpreted-text role="meth"} if necessary. Previously it was only called in two-argument !pow{.interpreted-text role="func"} and the binary power operator. :::

:::: note ::: title Note :::

If the right operand's type is a subclass of the left operand's type and that subclass provides a different implementation of 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. :::: :::::::

::: method object.__iadd__(self, other) object.__isub__(self, other) object.__imul__(self, other) object.__imatmul__(self, other) object.__itruediv__(self, other) object.__ifloordiv__(self, other) object.__imod__(self, other) object.__ipow__(self, other[, modulo]) object.__ilshift__(self, other) object.__irshift__(self, other) object.__iand__(self, other) object.__ixor__(self, other) object.__ior__(self, other)

These methods are called to implement the augmented arithmetic assignments (+=, -=, *=, @=, /=, //=, %=, **=, <<=, >>=, &=, ^=, |=). These methods should attempt to do the operation in-place (modifying self) and return the result (which could be, but does not have to be, self). If a specific method is not defined, or if that method returns NotImplemented{.interpreted-text role="data"}, the augmented assignment falls back to the normal methods. For instance, if x is an instance of a class with an __iadd__{.interpreted-text role="meth"} method, x += y is equivalent to x = x.__iadd__(y) . If __iadd__{.interpreted-text role="meth"} does not exist, or if x.__iadd__(y) returns !NotImplemented{.interpreted-text role="data"}, x.__add__(y) and y.__radd__(x) are considered, as with the evaluation of x + y. In certain situations, augmented assignment can result in unexpected errors (see faq-augmented-assignment-tuple-error{.interpreted-text role="ref"}), but this behavior is in fact part of the data model. :::

:::: method object.__neg__(self) object.__pos__(self) object.__abs__(self) object.__invert__(self)

::: index pair: built-in function; abs :::

Called to implement the unary arithmetic operations (-, +, abs{.interpreted-text role="func"} and ~). ::::

:::: method object.__complex__(self) object.__int__(self) object.__float__(self)

::: index pair: built-in function; complex pair: built-in function; int pair: built-in function; float :::

Called to implement the built-in functions complex{.interpreted-text role="func"}, int{.interpreted-text role="func"} and float{.interpreted-text role="func"}. Should return a value of the appropriate type. ::::

::: method object.__index__(self)

Called to implement operator.index{.interpreted-text role="func"}, and whenever Python needs to losslessly convert the numeric object to an integer object (such as in slicing, or in the built-in bin{.interpreted-text role="func"}, hex{.interpreted-text role="func"} and oct{.interpreted-text role="func"} functions). Presence of this method indicates that the numeric object is an integer type. Must return an integer.

If __int__{.interpreted-text role="meth"}, __float__{.interpreted-text role="meth"} and __complex__{.interpreted-text role="meth"} are not defined then corresponding built-in functions int{.interpreted-text role="func"}, float{.interpreted-text role="func"} and complex{.interpreted-text role="func"} fall back to __index__{.interpreted-text role="meth"}. :::

::::: method object.__round__(self, [,ndigits]) object.__trunc__(self) object.__floor__(self) object.__ceil__(self)

::: index pair: built-in function; round :::

Called to implement the built-in function round{.interpreted-text role="func"} and math{.interpreted-text role="mod"} functions ~math.trunc{.interpreted-text role="func"}, ~math.floor{.interpreted-text role="func"} and ~math.ceil{.interpreted-text role="func"}. Unless ndigits is passed to !__round__{.interpreted-text role="meth"} all these methods should return the value of the object truncated to an ~numbers.Integral{.interpreted-text role="class"} (typically an int{.interpreted-text role="class"}).

::: versionchanged 3.14 int{.interpreted-text role="func"} no longer delegates to the ~object.__trunc__{.interpreted-text role="meth"} method. ::: :::::

With Statement Context Managers {#context-managers}

A context manager{.interpreted-text role="dfn"} is an object that defines the runtime context to be established when executing a with{.interpreted-text role="keyword"} 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 !with{.interpreted-text role="keyword"} statement (described in section with{.interpreted-text role="ref"}), but can also be used by directly invoking their methods.

::: index pair: statement; with single: context manager :::

Typical uses of context managers include saving and restoring various kinds of global state, locking and unlocking resources, closing opened files, etc.

For more information on context managers, see typecontextmanager{.interpreted-text role="ref"}. The object{.interpreted-text role="class"} class itself does not provide the context manager methods.

::: method object.__enter__(self)

Enter the runtime context related to this object. The with{.interpreted-text role="keyword"} statement will bind this method's return value to the target(s) specified in the !as{.interpreted-text role="keyword"} clause of the statement, if any. :::

::: method object.__exit__(self, exc_type, exc_value, traceback)

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 None{.interpreted-text role="const"}.

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.

Note that ~object.__exit__{.interpreted-text role="meth"} methods should not reraise the passed-in exception; this is the caller's responsibility. :::

::: seealso

343{.interpreted-text role="pep"} - The "with" statement

: The specification, background, and examples for the Python with{.interpreted-text role="keyword"} statement. :::

Customizing positional arguments in class pattern matching {#class-pattern-matching}

When using a class name in a pattern, positional arguments in the pattern are not allowed by default, i.e. case MyClass(x, y) is typically invalid without special support in MyClass. To be able to use that kind of pattern, the class needs to define a __match_args__ attribute.

::: data object.__match_args__

This class variable can be assigned a tuple of strings. When this class is used in a class pattern with positional arguments, each positional argument will be converted into a keyword argument, using the corresponding value in __match_args__ as the keyword. The absence of this attribute is equivalent to setting it to (). :::

For example, if MyClass.__match_args__ is ("left", "center", "right") that means that case MyClass(x, y) is equivalent to case MyClass(left=x, center=y). Note that the number of arguments in the pattern must be smaller than or equal to the number of elements in __match_args__; if it is larger, the pattern match attempt will raise a TypeError{.interpreted-text role="exc"}.

::: versionadded 3.10 :::

::: seealso

634{.interpreted-text role="pep"} - Structural Pattern Matching

: The specification for the Python match statement. :::

Emulating buffer types {#python-buffer-protocol}

The buffer protocol <bufferobjects>{.interpreted-text role="ref"} provides a way for Python objects to expose efficient access to a low-level memory array. This protocol is implemented by builtin types such as bytes{.interpreted-text role="class"} and memoryview{.interpreted-text role="class"}, and third-party libraries may define additional buffer types.

While buffer types are usually implemented in C, it is also possible to implement the protocol in Python.

::: method object.__buffer__(self, flags)

Called when a buffer is requested from self (for example, by the memoryview{.interpreted-text role="class"} constructor). The flags argument is an integer representing the kind of buffer requested, affecting for example whether the returned buffer is read-only or writable. inspect.BufferFlags{.interpreted-text role="class"} provides a convenient way to interpret the flags. The method must return a memoryview{.interpreted-text role="class"} object. :::

::: method object.__release_buffer__(self, buffer)

Called when a buffer is no longer needed. The buffer argument is a memoryview{.interpreted-text role="class"} object that was previously returned by ~object.__buffer__{.interpreted-text role="meth"}. The method must release any resources associated with the buffer. This method should return None. Buffer objects that do not need to perform any cleanup are not required to implement this method. :::

::: versionadded 3.12 :::

::: seealso

688{.interpreted-text role="pep"} - Making the buffer protocol accessible in Python

: Introduces the Python __buffer__ and __release_buffer__ methods.

collections.abc.Buffer{.interpreted-text role="class"}

: ABC for buffer types. :::

Annotations

Functions, classes, and modules may contain annotations <annotation>{.interpreted-text role="term"}, which are a way to associate information (usually type hints <type hint>{.interpreted-text role="term"}) with a symbol.

:::: attribute object.__annotations__

This attribute contains the annotations for an object. It is lazily evaluated <lazy-evaluation>{.interpreted-text role="ref"}, so accessing the attribute may execute arbitrary code and raise exceptions. If evaluation is successful, the attribute is set to a dictionary mapping from variable names to annotations.

::: versionchanged 3.14 Annotations are now lazily evaluated. ::: ::::

:::: method object.__annotate__(format)

An annotate function{.interpreted-text role="term"}. Returns a new dictionary object mapping attribute/parameter names to their annotation values.

Takes a format parameter specifying the format in which annotations values should be provided. It must be a member of the annotationlib.Format{.interpreted-text role="class"} enum, or an integer with a value corresponding to a member of the enum.

If an annotate function doesn't support the requested format, it must raise NotImplementedError{.interpreted-text role="exc"}. Annotate functions must always support ~annotationlib.Format.VALUE{.interpreted-text role="attr"} format; they must not raise NotImplementedError(){.interpreted-text role="exc"} when called with this format.

When called with ~annotationlib.Format.VALUE{.interpreted-text role="attr"} format, an annotate function may raise NameError{.interpreted-text role="exc"}; it must not raise !NameError{.interpreted-text role="exc"} when called requesting any other format.

If an object does not have any annotations, ~object.__annotate__{.interpreted-text role="attr"} should preferably be set to None (it can't be deleted), rather than set to a function that returns an empty dict.

::: versionadded 3.14 ::: ::::

::: seealso

649{.interpreted-text role="pep"} --- Deferred evaluation of annotation using descriptors

: Introduces lazy evaluation of annotations and the __annotate__ function. :::

Special method lookup {#special-lookup}

For custom 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:

>>> class C:
...     pass
...
>>> c = C()
>>> c.__len__ = lambda: 5
>>> len(c)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'C' has no len()

The rationale behind this behaviour lies with a number of special methods such as ~object.__hash__{.interpreted-text role="meth"} and ~object.__repr__{.interpreted-text role="meth"} 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:

>>> 1 .__hash__() == hash(1)
True
>>> int.__hash__() == hash(int)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: descriptor '__hash__' of 'int' object needs an argument

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:

>>> type(1).__hash__(1) == hash(1)
True
>>> type(int).__hash__(int) == hash(int)
True

In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the ~object.__getattribute__{.interpreted-text role="meth"} method even of the object's metaclass:

>>> class Meta(type):
...     def __getattribute__(*args):
...         print("Metaclass getattribute invoked")
...         return type.__getattribute__(*args)
...
>>> class C(object, metaclass=Meta):
...     def __len__(self):
...         return 10
...     def __getattribute__(*args):
...         print("Class getattribute invoked")
...         return object.__getattribute__(*args)
...
>>> c = C()
>>> c.__len__()                 # Explicit lookup via instance
Class getattribute invoked
10
>>> type(c).__len__(c)          # Explicit lookup via type
Metaclass getattribute invoked
10
>>> len(c)                      # Implicit lookup
10

Bypassing the ~object.__getattribute__{.interpreted-text role="meth"} 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 must be set on the class object itself in order to be consistently invoked by the interpreter).

::: index single: coroutine :::

Coroutines

Awaitable Objects

An awaitable{.interpreted-text role="term"} object generally implements an ~object.__await__{.interpreted-text role="meth"} method. Coroutine objects <coroutine>{.interpreted-text role="term"} returned from async def{.interpreted-text role="keyword"} functions are awaitable.

:::: note ::: title Note :::

The generator iterator{.interpreted-text role="term"} objects returned from generators decorated with types.coroutine{.interpreted-text role="func"} are also awaitable, but they do not implement ~object.__await__{.interpreted-text role="meth"}. ::::

::::: method object.__await__(self)

Must return an iterator{.interpreted-text role="term"}. Should be used to implement awaitable{.interpreted-text role="term"} objects. For instance, asyncio.Future{.interpreted-text role="class"} implements this method to be compatible with the await{.interpreted-text role="keyword"} expression. The object{.interpreted-text role="class"} class itself is not awaitable and does not provide this method.

:::: note ::: title Note :::

The language doesn't place any restriction on the type or value of the objects yielded by the iterator returned by __await__, as this is specific to the implementation of the asynchronous execution framework (e.g. asyncio{.interpreted-text role="mod"}) that will be managing the awaitable{.interpreted-text role="term"} object. :::: :::::

::: versionadded 3.5 :::

::: seealso 492{.interpreted-text role="pep"} for additional information about awaitable objects. :::

Coroutine Objects

Coroutine objects <coroutine>{.interpreted-text role="term"} are awaitable{.interpreted-text role="term"} objects. A coroutine's execution can be controlled by calling ~object.__await__{.interpreted-text role="meth"} and iterating over the result. When the coroutine has finished executing and returns, the iterator raises StopIteration{.interpreted-text role="exc"}, and the exception's ~StopIteration.value{.interpreted-text role="attr"} attribute holds the return value. If the coroutine raises an exception, it is propagated by the iterator. Coroutines should not directly raise unhandled StopIteration{.interpreted-text role="exc"} exceptions.

Coroutines also have the methods listed below, which are analogous to those of generators (see generator-methods{.interpreted-text role="ref"}). However, unlike generators, coroutines do not directly support iteration.

::: versionchanged 3.5.2 It is a RuntimeError{.interpreted-text role="exc"} to await on a coroutine more than once. :::

::: method coroutine.send(value)

Starts or resumes execution of the coroutine. If value is None, this is equivalent to advancing the iterator returned by ~object.__await__{.interpreted-text role="meth"}. If value is not None, this method delegates to the ~generator.send{.interpreted-text role="meth"} method of the iterator that caused the coroutine to suspend. The result (return value, StopIteration{.interpreted-text role="exc"}, or other exception) is the same as when iterating over the !__await__{.interpreted-text role="meth"} return value, described above. :::

:::: method coroutine.throw(value) coroutine.throw(type[, value[, traceback]])

Raises the specified exception in the coroutine. This method delegates to the ~generator.throw{.interpreted-text role="meth"} method of the iterator that caused the coroutine to suspend, if it has such a method. Otherwise, the exception is raised at the suspension point. The result (return value, StopIteration{.interpreted-text role="exc"}, or other exception) is the same as when iterating over the ~object.__await__{.interpreted-text role="meth"} return value, described above. If the exception is not caught in the coroutine, it propagates back to the caller.

::: versionchanged 3.12

The second signature (type[, value[, traceback]]) is deprecated and may be removed in a future version of Python. ::: ::::

::: method coroutine.close()

Causes the coroutine to clean itself up and exit. If the coroutine is suspended, this method first delegates to the ~generator.close{.interpreted-text role="meth"} method of the iterator that caused the coroutine to suspend, if it has such a method. Then it raises GeneratorExit{.interpreted-text role="exc"} at the suspension point, causing the coroutine to immediately clean itself up. Finally, the coroutine is marked as having finished executing, even if it was never started.

Coroutine objects are automatically closed using the above process when they are about to be destroyed. :::

Asynchronous Iterators {#async-iterators}

An asynchronous iterator can call asynchronous code in its __anext__ method.

Asynchronous iterators can be used in an async for{.interpreted-text role="keyword"} statement.

The object{.interpreted-text role="class"} class itself does not provide these methods.

::: method object.__aiter__(self)

Must return an asynchronous iterator object. :::

::: method object.__anext__(self)

Must return an awaitable resulting in a next value of the iterator. Should raise a StopAsyncIteration{.interpreted-text role="exc"} error when the iteration is over. :::

An example of an asynchronous iterable object:

class Reader:
    async def readline(self):
        ...

    def __aiter__(self):
        return self

    async def __anext__(self):
        val = await self.readline()
        if val == b'':
            raise StopAsyncIteration
        return val

::: versionadded 3.5 :::

::: versionchanged 3.7 Prior to Python 3.7, ~object.__aiter__{.interpreted-text role="meth"} could return an awaitable that would resolve to an asynchronous iterator <asynchronous iterator>{.interpreted-text role="term"}.

Starting with Python 3.7, ~object.__aiter__{.interpreted-text role="meth"} must return an asynchronous iterator object. Returning anything else will result in a TypeError{.interpreted-text role="exc"} error. :::

Asynchronous Context Managers {#async-context-managers}

An asynchronous context manager is a context manager that is able to suspend execution in its __aenter__ and __aexit__ methods.

Asynchronous context managers can be used in an async with{.interpreted-text role="keyword"} statement.

The object{.interpreted-text role="class"} class itself does not provide these methods.

::: method object.__aenter__(self)

Semantically similar to ~object.__enter__{.interpreted-text role="meth"}, the only difference being that it must return an awaitable. :::

::: method object.__aexit__(self, exc_type, exc_value, traceback)

Semantically similar to ~object.__exit__{.interpreted-text role="meth"}, the only difference being that it must return an awaitable. :::

An example of an asynchronous context manager class:

class AsyncContextManager:
    async def __aenter__(self):
        await log('entering context')

    async def __aexit__(self, exc_type, exc, tb):
        await log('exiting context')

::: versionadded 3.5 :::

Footnotes

[^1]: It is 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.

[^2]: The ~object.__hash__{.interpreted-text role="meth"}, ~object.__iter__{.interpreted-text role="meth"}, ~object.__reversed__{.interpreted-text role="meth"}, ~object.__contains__{.interpreted-text role="meth"}, ~object.__class_getitem__{.interpreted-text role="meth"} and ~os.PathLike.__fspath__{.interpreted-text role="meth"} methods have special handling for this. Others will still raise a TypeError{.interpreted-text role="exc"}, but may do so by relying on the behavior that None is not callable.

[^3]: "Does not support" here means that the class has no such method, or the method returns NotImplemented{.interpreted-text role="data"}. Do not set the method to None if you want to force fallback to the right operand's reflected method---that will instead have the opposite effect of explicitly blocking such fallback.

[^4]: For operands of the same type, it is assumed that if the non-reflected method (such as ~object.__add__{.interpreted-text role="meth"}) fails then the operation is not supported, which is why the reflected method is not called.

[^5]: If the right operand's type is a subclass of the left operand's type, the reflected method having precedence allows subclasses to override their ancestors' operations.