: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.
The collections module offers the following :term:`ABCs <abstract base class>`:
.. tabularcolumns:: |l|L|L|L|
ABCs for classes that provide respectively the methods :meth:`__contains__`, :meth:`__hash__`, :meth:`__len__`, and :meth:`__call__`.
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).
ABC for sized iterable container classes.
.. versionadded:: 3.6
ABC for classes that provide the :meth:`~iterator.__iter__` and :meth:`~iterator.__next__` methods. See also the definition of :term:`iterator`.
ABC for iterable classes that also provide the :meth:`__reversed__` method.
.. versionadded:: 3.6
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
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.
ABCs for read-only and mutable sets.
ABCs for read-only and mutable :term:`mappings <mapping>`.
ABCs for mapping, items, keys, and values :term:`views <dictionary view>`.
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
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
ABC for classes that provide __aiter__ method. See also the
definition of :term:`asynchronous iterable`.
.. versionadded:: 3.5
ABC for classes that provide __aiter__ and __anext__
methods. See also the definition of :term:`asynchronous iterator`.
.. versionadded:: 3.5
ABC for asynchronous generator classes that implement the protocol defined in PEP 525 and PEP 492.
.. versionadded:: 3.6
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:
- 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 callscls(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. - 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.
- 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`.