:mod:`collections.abc` --- Abstract Base Classes for Containers =============================================================== .. module:: collections.abc :synopsis: Abstract base classes for containers .. moduleauthor:: Raymond Hettinger .. sectionauthor:: Raymond Hettinger .. versionadded:: 3.3 Formerly, this module was part of the :mod:`collections` module. .. testsetup:: * from collections import * import itertools __name__ = '' **Source code:** :source:`Lib/_collections_abc.py` -------------- This module provides :term:`abstract base classes ` 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 `: .. 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:`Sized` ``__len__`` :class:`Callable` ``__call__`` :class:`Sequence` :class:`Sized`, ``__getitem__``, ``__contains__``, ``__iter__``, ``__reversed__``, :class:`Iterable`, ``__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:`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:: 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:: Sequence MutableSequence ABCs for read-only and mutable :term:`sequences `. .. class:: Set MutableSet ABCs for read-only and mutable sets. .. class:: Mapping MutableMapping ABCs for read-only and mutable :term:`mappings `. .. class:: MappingView ItemsView KeysView ValuesView ABCs for mapping, items, keys, and values :term:`views `. 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 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 hashabilty using mixins, inherit from both :meth:`Set` and :meth:`Hashable`, then define ``__hash__ = Set._hash``. .. seealso:: * `OrderedSet recipe `_ for an example built on :class:`MutableSet`. * For more about ABCs, see the :mod:`abc` module and :pep:`3119`.