140 lines
6.8 KiB
ReStructuredText
140 lines
6.8 KiB
ReStructuredText
:mod:`collections.abc` --- Abstract Base Classes for Containers
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===============================================================
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.. module:: collections.abc
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:synopsis: Abstract base classes for containers
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.. moduleauthor:: Raymond Hettinger <python at rcn.com>
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.. sectionauthor:: Raymond Hettinger <python at rcn.com>
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.. testsetup:: *
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from collections import *
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import itertools
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__name__ = '<doctest>'
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**Source code:** :source:`Lib/collections/abc.py`
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--------------
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This module provides :term:`abstract base classes <abstract base class>` that
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can be used to test whether a class provides a particular interface; for
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example, whether it is hashable or whether it is a mapping.
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.. versionchanged:: 3.3
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Formerly, this module was part of the :mod:`collections` module.
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.. _abstract-base-classes:
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Collections Abstract Base Classes
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---------------------------------
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The collections module offers the following ABCs:
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========================= ===================== ====================== ====================================================
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ABC Inherits Abstract Methods Mixin Methods
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========================= ===================== ====================== ====================================================
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:class:`Container` ``__contains__``
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:class:`Hashable` ``__hash__``
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:class:`Iterable` ``__iter__``
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:class:`Iterator` :class:`Iterable` ``__next__`` ``__iter__``
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:class:`Sized` ``__len__``
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:class:`Callable` ``__call__``
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:class:`Sequence` :class:`Sized`, ``__getitem__`` ``__contains__``, ``__iter__``, ``__reversed__``,
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:class:`Iterable`, ``index``, and ``count``
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:class:`Container`
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:class:`MutableSequence` :class:`Sequence` ``__setitem__`` Inherited Sequence methods and
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``__delitem__``, ``append``, ``reverse``, ``extend``, ``pop``,
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and ``insert`` ``remove``, and ``__iadd__``
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:class:`Set` :class:`Sized`, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``,
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:class:`Iterable`, ``__gt__``, ``__ge__``, ``__and__``, ``__or__``,
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:class:`Container` ``__sub__``, ``__xor__``, and ``isdisjoint``
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:class:`MutableSet` :class:`Set` ``add`` and Inherited Set methods and
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``discard`` ``clear``, ``pop``, ``remove``, ``__ior__``,
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``__iand__``, ``__ixor__``, and ``__isub__``
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:class:`Mapping` :class:`Sized`, ``__getitem__`` ``__contains__``, ``keys``, ``items``, ``values``,
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:class:`Iterable`, ``get``, ``__eq__``, and ``__ne__``
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:class:`Container`
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:class:`MutableMapping` :class:`Mapping` ``__setitem__`` and Inherited Mapping methods and
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``__delitem__`` ``pop``, ``popitem``, ``clear``, ``update``,
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and ``setdefault``
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:class:`MappingView` :class:`Sized` ``__len__``
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:class:`KeysView` :class:`MappingView`, ``__contains__``,
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:class:`Set` ``__iter__``
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:class:`ItemsView` :class:`MappingView`, ``__contains__``,
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:class:`Set` ``__iter__``
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:class:`ValuesView` :class:`MappingView` ``__contains__``, ``__iter__``
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========================= ===================== ====================== ====================================================
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These ABCs allow us to ask classes or instances if they provide
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particular functionality, for example::
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size = None
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if isinstance(myvar, collections.Sized):
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size = len(myvar)
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Several of the ABCs are also useful as mixins that make it easier to develop
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classes supporting container APIs. For example, to write a class supporting
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the full :class:`Set` API, it only necessary to supply the three underlying
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abstract methods: :meth:`__contains__`, :meth:`__iter__`, and :meth:`__len__`.
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The ABC supplies the remaining methods such as :meth:`__and__` and
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:meth:`isdisjoint` ::
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class ListBasedSet(collections.Set):
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''' Alternate set implementation favoring space over speed
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and not requiring the set elements to be hashable. '''
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def __init__(self, iterable):
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self.elements = lst = []
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for value in iterable:
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if value not in lst:
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lst.append(value)
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def __iter__(self):
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return iter(self.elements)
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def __contains__(self, value):
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return value in self.elements
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def __len__(self):
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return len(self.elements)
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s1 = ListBasedSet('abcdef')
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s2 = ListBasedSet('defghi')
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overlap = s1 & s2 # The __and__() method is supported automatically
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Notes on using :class:`Set` and :class:`MutableSet` as a mixin:
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(1)
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Since some set operations create new sets, the default mixin methods need
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a way to create new instances from an iterable. The class constructor is
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assumed to have a signature in the form ``ClassName(iterable)``.
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That assumption is factored-out to an internal classmethod called
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:meth:`_from_iterable` which calls ``cls(iterable)`` to produce a new set.
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If the :class:`Set` mixin is being used in a class with a different
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constructor signature, you will need to override :meth:`from_iterable`
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with a classmethod that can construct new instances from
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an iterable argument.
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(2)
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To override the comparisons (presumably for speed, as the
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semantics are fixed), redefine :meth:`__le__` and
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then the other operations will automatically follow suit.
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(3)
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The :class:`Set` mixin provides a :meth:`_hash` method to compute a hash value
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for the set; however, :meth:`__hash__` is not defined because not all sets
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are hashable or immutable. To add set hashabilty using mixins,
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inherit from both :meth:`Set` and :meth:`Hashable`, then define
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``__hash__ = Set._hash``.
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.. seealso::
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* `OrderedSet recipe <http://code.activestate.com/recipes/576694/>`_ that uses
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:class:`MutableSet`.
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* For more about ABCs, see the :mod:`abc` module and :pep:`3119`.
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