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

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.

Checking isinstance(obj, Iterable) detects classes that are registered as Iterable or that have an __iter__() method, but it does not detect classes that iterate with the __getitem__() method. The only reliable way to determine whether an object is iterable is to call iter(obj).

class collections.abc.Collection

ABC for sized iterable container classes.

New in version 3.6.

class collections.abc.Iterator

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

class collections.abc.Reversible

ABC for iterable classes that also provide the __reversed__() method.

New in version 3.6.

class collections.abc.Generator

ABC for generator classes that implement the protocol defined in PEP 342 that extends iterators with the send(), throw() and close() methods. See also the definition of generator.

New in version 3.5.

class collections.abc.Sequence

class collections.abc.MutableSequence

class collections.abc.ByteString

ABCs for read-only and mutable sequences.

Implementation note: Some of the mixin methods, such as __iter__(), __reversed__() and index(), make repeated calls to the underlying __getitem__() method. Consequently, if __getitem__() is implemented with constant access speed, the mixin methods will have linear performance; however, if the underlying method is linear (as it would be with a linked list), the mixins will have quadratic performance and will likely need to be overridden.

Changed in version 3.5: The index() method added support for stop and start arguments.

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.

class collections.abc.Awaitable

ABC for awaitable objects, which can be used in await expressions. Custom implementations must provide the __await__() method.

Coroutine objects and instances of the Coroutine ABC are all instances of this ABC.

Note

In CPython, generator-based coroutines (generators decorated with types.coroutine() or asyncio.coroutine()) are awaitables, even though they do not have an __await__() method. Using isinstance(gencoro, Awaitable) for them will return False. Use inspect.isawaitable() to detect them.

New in version 3.5.

class collections.abc.Coroutine

ABC for coroutine compatible classes. These implement the following methods, defined in Coroutine Objects: send(), throw(), and close(). Custom implementations must also implement __await__(). All Coroutine instances are also instances of Awaitable. See also the definition of coroutine.

Note

In CPython, generator-based coroutines (generators decorated with types.coroutine() or asyncio.coroutine()) are awaitables, even though they do not have an __await__() method. Using isinstance(gencoro, Coroutine) for them will return False. Use inspect.isawaitable() to detect them.

New in version 3.5.

class collections.abc.AsyncIterable

ABC for classes that provide __aiter__ method. See also the definition of asynchronous iterable.

New in version 3.5.

class collections.abc.AsyncIterator

ABC for classes that provide __aiter__ and __anext__ methods. See also the definition of asynchronous iterator.

New in version 3.5.

class collections.abc.AsyncGenerator

ABC for asynchronous generator classes that implement the protocol defined in PEP 525 and PEP 492.

New in version 3.6.

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 is 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 __ge__(), 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 hashability using mixins, inherit from both Set() and Hashable(), then define __hash__ = Set._hash.