diff options
Diffstat (limited to 'Doc/library/collections.abc.rst')
| -rw-r--r-- | Doc/library/collections.abc.rst | 139 |
1 files changed, 139 insertions, 0 deletions
diff --git a/Doc/library/collections.abc.rst b/Doc/library/collections.abc.rst new file mode 100644 index 0000000..fc6bbbf --- /dev/null +++ b/Doc/library/collections.abc.rst @@ -0,0 +1,139 @@ +: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> + +.. testsetup:: * + + from collections import * + import itertools + __name__ = '<doctest>' + +**Source code:** :source:`Lib/collections/abc.py` + +-------------- + +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. + +.. versionchanged:: 3.3 + Formerly, this module was part of the :mod:`collections` module. + +.. _abstract-base-classes: + +Collections Abstract Base Classes +--------------------------------- + +The collections module offers the following ABCs: + +========================= ===================== ====================== ==================================================== +ABC Inherits 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`, ``index``, and ``count`` + :class:`Container` + +:class:`MutableSequence` :class:`Sequence` ``__setitem__`` Inherited Sequence methods and + ``__delitem__``, ``append``, ``reverse``, ``extend``, ``pop``, + and ``insert`` ``remove``, ``clear``, and ``__iadd__`` + +:class:`Set` :class:`Sized`, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``, + :class:`Iterable`, ``__gt__``, ``__ge__``, ``__and__``, ``__or__``, + :class:`Container` ``__sub__``, ``__xor__``, and ``isdisjoint`` + +:class:`MutableSet` :class:`Set` ``add`` and Inherited Set methods and + ``discard`` ``clear``, ``pop``, ``remove``, ``__ior__``, + ``__iand__``, ``__ixor__``, and ``__isub__`` + +:class:`Mapping` :class:`Sized`, ``__getitem__`` ``__contains__``, ``keys``, ``items``, ``values``, + :class:`Iterable`, ``get``, ``__eq__``, and ``__ne__`` + :class:`Container` + +:class:`MutableMapping` :class:`Mapping` ``__setitem__`` and Inherited Mapping methods and + ``__delitem__`` ``pop``, ``popitem``, ``clear``, ``update``, + and ``setdefault`` + + +:class:`MappingView` :class:`Sized` ``__len__`` +:class:`KeysView` :class:`MappingView`, ``__contains__``, + :class:`Set` ``__iter__`` +:class:`ItemsView` :class:`MappingView`, ``__contains__``, + :class:`Set` ``__iter__`` +:class:`ValuesView` :class:`MappingView` ``__contains__``, ``__iter__`` +========================= ===================== ====================== ==================================================== + +These ABCs allow us to ask classes or instances if they provide +particular functionality, for example:: + + size = None + if isinstance(myvar, collections.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.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 + 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 <http://code.activestate.com/recipes/576694/>`_ that uses + :class:`MutableSet`. + + * For more about ABCs, see the :mod:`abc` module and :pep:`3119`. |