8.4.1. Collections Abstract Base Classes

The collections module offers the following ABCs:

class collections.abc.Container

class collections.abc.Hashable

class collections.abc.Sized

class collections.abc.Callable

ABCs for classes that provide respectively the methods __contains__(), __hash__(), __len__(), and __call__().

class collections.abc.Iterable

ABC for classes that provide the __iter__() method. See also the definition of iterable.

class collections.abc.Iterator

ABC for classes that provide the __iter__() and __next__() methods. See also the definition of iterator.

class collections.abc.Sequence

class collections.abc.MutableSequence

ABCs for read-only and mutable sequences.

class collections.abc.Set

class collections.abc.MutableSet

ABCs for read-only and mutable sets.

class collections.abc.Mapping

class collections.abc.MutableMapping

ABCs for read-only and mutable mappings.

class collections.abc.MappingView

class collections.abc.ItemsView

class collections.abc.KeysView

class collections.abc.ValuesView

ABCs for mapping, items, keys, and values 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 Set API, it only necessary to supply the three underlying abstract methods: __contains__(), __iter__(), and __len__(). The ABC supplies the remaining methods such as __and__() and 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 Set and 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 _from_iterable() which calls cls(iterable) to produce a new set. If the Set mixin is being used in a class with a different constructor signature, you will need to override _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 __le__() and then the other operations will automatically follow suit.
3. The Set mixin provides a _hash() method to compute a hash value for the set; however, __hash__() is not defined because not all sets are hashable or immutable. To add set hashabilty using mixins, inherit from both Set() and Hashable(), then define __hash__ = Set._hash.