mirror of https://github.com/python/cpython
433 lines
17 KiB
ReStructuredText
433 lines
17 KiB
ReStructuredText
:mod:`collections.abc` --- Abstract Base Classes for Containers
|
|
===============================================================
|
|
|
|
.. module:: collections.abc
|
|
:synopsis: Abstract base classes for containers
|
|
|
|
.. moduleauthor:: Raymond Hettinger <python at rcn.com>
|
|
.. sectionauthor:: Raymond Hettinger <python at rcn.com>
|
|
|
|
.. versionadded:: 3.3
|
|
Formerly, this module was part of the :mod:`collections` module.
|
|
|
|
**Source code:** :source:`Lib/_collections_abc.py`
|
|
|
|
.. testsetup:: *
|
|
|
|
from collections.abc import *
|
|
import itertools
|
|
__name__ = '<doctest>'
|
|
|
|
--------------
|
|
|
|
This module provides :term:`abstract base classes <abstract base class>` that
|
|
can be used to test whether a class provides a particular interface; for
|
|
example, whether it is :term:`hashable` or whether it is a mapping.
|
|
|
|
An :func:`issubclass` or :func:`isinstance` test for an interface works in one
|
|
of three ways.
|
|
|
|
1) A newly written class can inherit directly from one of the
|
|
abstract base classes. The class must supply the required abstract
|
|
methods. The remaining mixin methods come from inheritance and can be
|
|
overridden if desired. Other methods may be added as needed:
|
|
|
|
.. testcode::
|
|
|
|
class C(Sequence): # Direct inheritance
|
|
def __init__(self): ... # Extra method not required by the ABC
|
|
def __getitem__(self, index): ... # Required abstract method
|
|
def __len__(self): ... # Required abstract method
|
|
def count(self, value): ... # Optionally override a mixin method
|
|
|
|
.. doctest::
|
|
|
|
>>> issubclass(C, Sequence)
|
|
True
|
|
>>> isinstance(C(), Sequence)
|
|
True
|
|
|
|
2) Existing classes and built-in classes can be registered as "virtual
|
|
subclasses" of the ABCs. Those classes should define the full API
|
|
including all of the abstract methods and all of the mixin methods.
|
|
This lets users rely on :func:`issubclass` or :func:`isinstance` tests
|
|
to determine whether the full interface is supported. The exception to
|
|
this rule is for methods that are automatically inferred from the rest
|
|
of the API:
|
|
|
|
.. testcode::
|
|
|
|
class D: # No inheritance
|
|
def __init__(self): ... # Extra method not required by the ABC
|
|
def __getitem__(self, index): ... # Abstract method
|
|
def __len__(self): ... # Abstract method
|
|
def count(self, value): ... # Mixin method
|
|
def index(self, value): ... # Mixin method
|
|
|
|
Sequence.register(D) # Register instead of inherit
|
|
|
|
.. doctest::
|
|
|
|
>>> issubclass(D, Sequence)
|
|
True
|
|
>>> isinstance(D(), Sequence)
|
|
True
|
|
|
|
In this example, class :class:`D` does not need to define
|
|
``__contains__``, ``__iter__``, and ``__reversed__`` because the
|
|
:ref:`in-operator <comparisons>`, the :term:`iteration <iterable>`
|
|
logic, and the :func:`reversed` function automatically fall back to
|
|
using ``__getitem__`` and ``__len__``.
|
|
|
|
3) Some simple interfaces are directly recognizable by the presence of
|
|
the required methods (unless those methods have been set to
|
|
:const:`None`):
|
|
|
|
.. testcode::
|
|
|
|
class E:
|
|
def __iter__(self): ...
|
|
def __next__(next): ...
|
|
|
|
.. doctest::
|
|
|
|
>>> issubclass(E, Iterable)
|
|
True
|
|
>>> isinstance(E(), Iterable)
|
|
True
|
|
|
|
Complex interfaces do not support this last technique because an
|
|
interface is more than just the presence of method names. Interfaces
|
|
specify semantics and relationships between methods that cannot be
|
|
inferred solely from the presence of specific method names. For
|
|
example, knowing that a class supplies ``__getitem__``, ``__len__``, and
|
|
``__iter__`` is insufficient for distinguishing a :class:`Sequence` from
|
|
a :class:`Mapping`.
|
|
|
|
.. versionadded:: 3.9
|
|
These abstract classes now support ``[]``. See :ref:`types-genericalias`
|
|
and :pep:`585`.
|
|
|
|
.. _collections-abstract-base-classes:
|
|
|
|
Collections Abstract Base Classes
|
|
---------------------------------
|
|
|
|
The collections module offers the following :term:`ABCs <abstract base class>`:
|
|
|
|
.. tabularcolumns:: |l|L|L|L|
|
|
|
|
============================== ====================== ======================= ====================================================
|
|
ABC Inherits from Abstract Methods Mixin Methods
|
|
============================== ====================== ======================= ====================================================
|
|
:class:`Container` [1]_ ``__contains__``
|
|
:class:`Hashable` [1]_ ``__hash__``
|
|
:class:`Iterable` [1]_ [2]_ ``__iter__``
|
|
:class:`Iterator` [1]_ :class:`Iterable` ``__next__`` ``__iter__``
|
|
:class:`Reversible` [1]_ :class:`Iterable` ``__reversed__``
|
|
:class:`Generator` [1]_ :class:`Iterator` ``send``, ``throw`` ``close``, ``__iter__``, ``__next__``
|
|
:class:`Sized` [1]_ ``__len__``
|
|
:class:`Callable` [1]_ ``__call__``
|
|
:class:`Collection` [1]_ :class:`Sized`, ``__contains__``,
|
|
:class:`Iterable`, ``__iter__``,
|
|
:class:`Container` ``__len__``
|
|
|
|
:class:`Sequence` :class:`Reversible`, ``__getitem__``, ``__contains__``, ``__iter__``, ``__reversed__``,
|
|
:class:`Collection` ``__len__`` ``index``, and ``count``
|
|
|
|
:class:`MutableSequence` :class:`Sequence` ``__getitem__``, Inherited :class:`Sequence` methods and
|
|
``__setitem__``, ``append``, ``reverse``, ``extend``, ``pop``,
|
|
``__delitem__``, ``remove``, and ``__iadd__``
|
|
``__len__``,
|
|
``insert``
|
|
|
|
:class:`ByteString` :class:`Sequence` ``__getitem__``, Inherited :class:`Sequence` methods
|
|
``__len__``
|
|
|
|
:class:`Set` :class:`Collection` ``__contains__``, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``,
|
|
``__iter__``, ``__gt__``, ``__ge__``, ``__and__``, ``__or__``,
|
|
``__len__`` ``__sub__``, ``__xor__``, and ``isdisjoint``
|
|
|
|
:class:`MutableSet` :class:`Set` ``__contains__``, Inherited :class:`Set` methods and
|
|
``__iter__``, ``clear``, ``pop``, ``remove``, ``__ior__``,
|
|
``__len__``, ``__iand__``, ``__ixor__``, and ``__isub__``
|
|
``add``,
|
|
``discard``
|
|
|
|
:class:`Mapping` :class:`Collection` ``__getitem__``, ``__contains__``, ``keys``, ``items``, ``values``,
|
|
``__iter__``, ``get``, ``__eq__``, and ``__ne__``
|
|
``__len__``
|
|
|
|
:class:`MutableMapping` :class:`Mapping` ``__getitem__``, Inherited :class:`Mapping` methods and
|
|
``__setitem__``, ``pop``, ``popitem``, ``clear``, ``update``,
|
|
``__delitem__``, and ``setdefault``
|
|
``__iter__``,
|
|
``__len__``
|
|
|
|
|
|
:class:`MappingView` :class:`Sized` ``__len__``
|
|
:class:`ItemsView` :class:`MappingView`, ``__contains__``,
|
|
:class:`Set` ``__iter__``
|
|
:class:`KeysView` :class:`MappingView`, ``__contains__``,
|
|
:class:`Set` ``__iter__``
|
|
:class:`ValuesView` :class:`MappingView`, ``__contains__``, ``__iter__``
|
|
:class:`Collection`
|
|
:class:`Awaitable` [1]_ ``__await__``
|
|
:class:`Coroutine` [1]_ :class:`Awaitable` ``send``, ``throw`` ``close``
|
|
:class:`AsyncIterable` [1]_ ``__aiter__``
|
|
:class:`AsyncIterator` [1]_ :class:`AsyncIterable` ``__anext__`` ``__aiter__``
|
|
:class:`AsyncGenerator` [1]_ :class:`AsyncIterator` ``asend``, ``athrow`` ``aclose``, ``__aiter__``, ``__anext__``
|
|
:class:`Buffer` [1]_ ``__buffer__``
|
|
============================== ====================== ======================= ====================================================
|
|
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [1] These ABCs override :meth:`object.__subclasshook__` to support
|
|
testing an interface by verifying the required methods are present
|
|
and have not been set to :const:`None`. This only works for simple
|
|
interfaces. More complex interfaces require registration or direct
|
|
subclassing.
|
|
|
|
.. [2] Checking ``isinstance(obj, Iterable)`` detects classes that are
|
|
registered as :class:`Iterable` or that have an :meth:`__iter__`
|
|
method, but it does not detect classes that iterate with the
|
|
:meth:`__getitem__` method. The only reliable way to determine
|
|
whether an object is :term:`iterable` is to call ``iter(obj)``.
|
|
|
|
|
|
Collections Abstract Base Classes -- Detailed Descriptions
|
|
----------------------------------------------------------
|
|
|
|
|
|
.. class:: Container
|
|
|
|
ABC for classes that provide the :meth:`__contains__` method.
|
|
|
|
.. class:: Hashable
|
|
|
|
ABC for classes that provide the :meth:`__hash__` method.
|
|
|
|
.. class:: Sized
|
|
|
|
ABC for classes that provide the :meth:`__len__` method.
|
|
|
|
.. class:: Callable
|
|
|
|
ABC for classes that provide the :meth:`__call__` method.
|
|
|
|
.. class:: Iterable
|
|
|
|
ABC for classes that provide the :meth:`__iter__` method.
|
|
|
|
Checking ``isinstance(obj, Iterable)`` detects classes that are registered
|
|
as :class:`Iterable` or that have an :meth:`__iter__` method, but it does
|
|
not detect classes that iterate with the :meth:`__getitem__` method.
|
|
The only reliable way to determine whether an object is :term:`iterable`
|
|
is to call ``iter(obj)``.
|
|
|
|
.. class:: Collection
|
|
|
|
ABC for sized iterable container classes.
|
|
|
|
.. versionadded:: 3.6
|
|
|
|
.. class:: Iterator
|
|
|
|
ABC for classes that provide the :meth:`~iterator.__iter__` and
|
|
:meth:`~iterator.__next__` methods. See also the definition of
|
|
:term:`iterator`.
|
|
|
|
.. class:: Reversible
|
|
|
|
ABC for iterable classes that also provide the :meth:`__reversed__`
|
|
method.
|
|
|
|
.. versionadded:: 3.6
|
|
|
|
.. class:: Generator
|
|
|
|
ABC for generator classes that implement the protocol defined in
|
|
:pep:`342` that extends iterators with the :meth:`~generator.send`,
|
|
:meth:`~generator.throw` and :meth:`~generator.close` methods.
|
|
See also the definition of :term:`generator`.
|
|
|
|
.. versionadded:: 3.5
|
|
|
|
.. class:: Sequence
|
|
MutableSequence
|
|
ByteString
|
|
|
|
ABCs for read-only and mutable :term:`sequences <sequence>`.
|
|
|
|
Implementation note: Some of the mixin methods, such as
|
|
:meth:`__iter__`, :meth:`__reversed__` and :meth:`index`, make
|
|
repeated calls to the underlying :meth:`__getitem__` method.
|
|
Consequently, if :meth:`__getitem__` is implemented with constant
|
|
access speed, the mixin methods will have linear performance;
|
|
however, if the underlying method is linear (as it would be with a
|
|
linked list), the mixins will have quadratic performance and will
|
|
likely need to be overridden.
|
|
|
|
.. versionchanged:: 3.5
|
|
The index() method added support for *stop* and *start*
|
|
arguments.
|
|
|
|
.. deprecated-removed:: 3.12 3.14
|
|
The :class:`ByteString` ABC has been deprecated.
|
|
For use in typing, prefer a union, like ``bytes | bytearray``, or
|
|
:class:`collections.abc.Buffer`.
|
|
For use as an ABC, prefer :class:`Sequence` or :class:`collections.abc.Buffer`.
|
|
|
|
.. class:: Set
|
|
MutableSet
|
|
|
|
ABCs for read-only and mutable sets.
|
|
|
|
.. class:: Mapping
|
|
MutableMapping
|
|
|
|
ABCs for read-only and mutable :term:`mappings <mapping>`.
|
|
|
|
.. class:: MappingView
|
|
ItemsView
|
|
KeysView
|
|
ValuesView
|
|
|
|
ABCs for mapping, items, keys, and values :term:`views <dictionary view>`.
|
|
|
|
.. class:: Awaitable
|
|
|
|
ABC for :term:`awaitable` objects, which can be used in :keyword:`await`
|
|
expressions. Custom implementations must provide the :meth:`__await__`
|
|
method.
|
|
|
|
:term:`Coroutine <coroutine>` objects and instances of the
|
|
:class:`~collections.abc.Coroutine` ABC are all instances of this ABC.
|
|
|
|
.. note::
|
|
In CPython, generator-based coroutines (generators decorated with
|
|
:func:`types.coroutine`) are
|
|
*awaitables*, even though they do not have an :meth:`__await__` method.
|
|
Using ``isinstance(gencoro, Awaitable)`` for them will return ``False``.
|
|
Use :func:`inspect.isawaitable` to detect them.
|
|
|
|
.. versionadded:: 3.5
|
|
|
|
.. class:: Coroutine
|
|
|
|
ABC for coroutine compatible classes. These implement the
|
|
following methods, defined in :ref:`coroutine-objects`:
|
|
:meth:`~coroutine.send`, :meth:`~coroutine.throw`, and
|
|
:meth:`~coroutine.close`. Custom implementations must also implement
|
|
:meth:`__await__`. All :class:`Coroutine` instances are also instances of
|
|
:class:`Awaitable`. See also the definition of :term:`coroutine`.
|
|
|
|
.. note::
|
|
In CPython, generator-based coroutines (generators decorated with
|
|
:func:`types.coroutine`) are
|
|
*awaitables*, even though they do not have an :meth:`__await__` method.
|
|
Using ``isinstance(gencoro, Coroutine)`` for them will return ``False``.
|
|
Use :func:`inspect.isawaitable` to detect them.
|
|
|
|
.. versionadded:: 3.5
|
|
|
|
.. class:: AsyncIterable
|
|
|
|
ABC for classes that provide ``__aiter__`` method. See also the
|
|
definition of :term:`asynchronous iterable`.
|
|
|
|
.. versionadded:: 3.5
|
|
|
|
.. class:: AsyncIterator
|
|
|
|
ABC for classes that provide ``__aiter__`` and ``__anext__``
|
|
methods. See also the definition of :term:`asynchronous iterator`.
|
|
|
|
.. versionadded:: 3.5
|
|
|
|
.. class:: AsyncGenerator
|
|
|
|
ABC for asynchronous generator classes that implement the protocol
|
|
defined in :pep:`525` and :pep:`492`.
|
|
|
|
.. versionadded:: 3.6
|
|
|
|
.. class:: Buffer
|
|
|
|
ABC for classes that provide the :meth:`~object.__buffer__` method,
|
|
implementing the :ref:`buffer protocol <bufferobjects>`. See :pep:`688`.
|
|
|
|
.. versionadded:: 3.12
|
|
|
|
Examples and Recipes
|
|
--------------------
|
|
|
|
ABCs allow us to ask classes or instances if they provide
|
|
particular functionality, for example::
|
|
|
|
size = None
|
|
if isinstance(myvar, collections.abc.Sized):
|
|
size = len(myvar)
|
|
|
|
Several of the ABCs are also useful as mixins that make it easier to develop
|
|
classes supporting container APIs. For example, to write a class supporting
|
|
the full :class:`Set` API, it is only necessary to supply the three underlying
|
|
abstract methods: :meth:`__contains__`, :meth:`__iter__`, and :meth:`__len__`.
|
|
The ABC supplies the remaining methods such as :meth:`__and__` and
|
|
:meth:`isdisjoint`::
|
|
|
|
class ListBasedSet(collections.abc.Set):
|
|
''' Alternate set implementation favoring space over speed
|
|
and not requiring the set elements to be hashable. '''
|
|
def __init__(self, iterable):
|
|
self.elements = lst = []
|
|
for value in iterable:
|
|
if value not in lst:
|
|
lst.append(value)
|
|
|
|
def __iter__(self):
|
|
return iter(self.elements)
|
|
|
|
def __contains__(self, value):
|
|
return value in self.elements
|
|
|
|
def __len__(self):
|
|
return len(self.elements)
|
|
|
|
s1 = ListBasedSet('abcdef')
|
|
s2 = ListBasedSet('defghi')
|
|
overlap = s1 & s2 # The __and__() method is supported automatically
|
|
|
|
Notes on using :class:`Set` and :class:`MutableSet` as a mixin:
|
|
|
|
(1)
|
|
Since some set operations create new sets, the default mixin methods need
|
|
a way to create new instances from an iterable. The class constructor is
|
|
assumed to have a signature in the form ``ClassName(iterable)``.
|
|
That assumption is factored-out to an internal classmethod called
|
|
:meth:`_from_iterable` which calls ``cls(iterable)`` to produce a new set.
|
|
If the :class:`Set` mixin is being used in a class with a different
|
|
constructor signature, you will need to override :meth:`_from_iterable`
|
|
with a classmethod or regular method that can construct new instances from
|
|
an iterable argument.
|
|
|
|
(2)
|
|
To override the comparisons (presumably for speed, as the
|
|
semantics are fixed), redefine :meth:`__le__` and :meth:`__ge__`,
|
|
then the other operations will automatically follow suit.
|
|
|
|
(3)
|
|
The :class:`Set` mixin provides a :meth:`_hash` method to compute a hash value
|
|
for the set; however, :meth:`__hash__` is not defined because not all sets
|
|
are :term:`hashable` or immutable. To add set hashability using mixins,
|
|
inherit from both :meth:`Set` and :meth:`Hashable`, then define
|
|
``__hash__ = Set._hash``.
|
|
|
|
.. seealso::
|
|
|
|
* `OrderedSet recipe <https://code.activestate.com/recipes/576694/>`_ for an
|
|
example built on :class:`MutableSet`.
|
|
|
|
* For more about ABCs, see the :mod:`abc` module and :pep:`3119`.
|