New in version 3.3:Formerly, this module was part of thecollections module.

8.4.1. Collections Abstract Base Classes¶

The collections module offers the followingABCs:

classcollections.abc.Container¶

classcollections.abc.Hashable¶

classcollections.abc.Sized¶

classcollections.abc.Callable¶

ABCs for classes that provide respectively the methods__contains__(),__hash__(),__len__(), and__call__().

classcollections.abc.Iterable¶

ABC for classes that provide the__iter__() method.See also the definition ofiterable.

classcollections.abc.Iterator¶

ABC for classes that provide the__iter__() and__next__() methods. See also the definition ofiterator.

classcollections.abc.Sequence¶

classcollections.abc.MutableSequence¶

ABCs for read-only and mutablesequences.

classcollections.abc.Set¶

classcollections.abc.MutableSet¶

ABCs for read-only and mutable sets.

classcollections.abc.Mapping¶

classcollections.abc.MutableMapping¶

ABCs for read-only and mutablemappings.

classcollections.abc.MappingView¶

classcollections.abc.ItemsView¶

classcollections.abc.KeysView¶

classcollections.abc.ValuesView¶

ABCs for mapping, items, keys, and valuesviews.

These ABCs allow us to ask classes or instances if they provideparticular functionality, for example:

size=Noneifisinstance(myvar,collections.abc.Sized):size=len(myvar)

Several of the ABCs are also useful as mixins that make it easier to developclasses supporting container APIs. For example, to write a class supportingthe fullSet API, it only necessary to supply the three underlyingabstract methods:__contains__(),__iter__(), and__len__().The ABC supplies the remaining methods such as__and__() andisdisjoint():

classListBasedSet(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=[]forvalueiniterable:ifvaluenotinlst:lst.append(value)def__iter__(self):returniter(self.elements)def__contains__(self,value):returnvalueinself.elementsdef__len__(self):returnlen(self.elements)s1=ListBasedSet('abcdef')s2=ListBasedSet('defghi')overlap=s1&s2# The __and__() method is supported automatically

Notes on usingSet andMutableSet as a mixin:

1. Since some set operations create new sets, the default mixin methods needa way to create new instances from an iterable. The class constructor isassumed to have a signature in the formClassName(iterable).That assumption is factored-out to an internal classmethod called_from_iterable() which callscls(iterable) to produce a new set.If theSet mixin is being used in a class with a differentconstructor signature, you will need to override_from_iterable()with a classmethod that can construct new instances froman iterable argument.
2. To override the comparisons (presumably for speed, as thesemantics are fixed), redefine__le__() andthen the other operations will automatically follow suit.
3. TheSet mixin provides a_hash() method to compute a hash valuefor the set; however,__hash__() is not defined because not all setsare hashable or immutable. To add set hashabilty using mixins,inherit from bothSet() andHashable(), then define__hash__=Set._hash.