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: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 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 hashable or whether it is a mapping.
.. _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` ``__contains__``
:class:`Hashable` ``__hash__``
:class:`Iterable` ``__iter__``
:class:`Iterator` :class:`Iterable` ``__next__`` ``__iter__``
:class:`Reversible` :class:`Iterable` ``__reversed__``
:class:`Generator` :class:`Iterator` ``send``, ``throw`` ``close``, ``__iter__``, ``__next__``
:class:`Sized` ``__len__``
:class:`Callable` ``__call__``
:class:`Sequence` :class:`Sized`, ``__getitem__``, ``__contains__``, ``__iter__``, ``__reversed__``,
:class:`Reversible`, ``__len__`` ``index``, and ``count``
:class:`Container`
: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:`Sized`, ``__contains__``, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``,
:class:`Iterable`, ``__iter__``, ``__gt__``, ``__ge__``, ``__and__``, ``__or__``,
:class:`Container` ``__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:`Sized`, ``__getitem__``, ``__contains__``, ``keys``, ``items``, ``values``,
:class:`Iterable`, ``__iter__``, ``get``, ``__eq__``, and ``__ne__``
:class:`Container` ``__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:`Awaitable` ``__await__``
:class:`Coroutine` :class:`Awaitable` ``send``, ``throw`` ``close``
:class:`AsyncIterable` ``__aiter__``
:class:`AsyncIterator` :class:`AsyncIterable` ``__anext__`` ``__aiter__``
========================== ====================== ======================= ====================================================
.. class:: Container
Hashable
Sized
Callable
ABCs for classes that provide respectively the methods :meth:`__contains__`,
:meth:`__hash__`, :meth:`__len__`, and :meth:`__call__`.
.. class:: Iterable
ABC for classes that provide the :meth:`__iter__` method.
See also the definition of :term:`iterable`.
.. 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.
.. 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` 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` or :func:`asyncio.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` or :func:`asyncio.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
These 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 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 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`.
|
