collections.abc — Abstract Base Classes for Containers — Python 3.8.20 documentation (original) (raw)

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

Source code: Lib/_collections_abc.py

This module provides abstract base classes 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.

Collections Abstract Base Classes¶

The collections module offers the following ABCs:

class collections.abc. Container¶

ABC for classes that provide the __contains__() method.

class collections.abc. Hashable¶

ABC for classes that provide the __hash__() method.

class collections.abc. Sized¶

ABC for classes that provide the __len__() method.

class collections.abc. Callable¶

ABC for classes that provide the __call__() method.

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 iterableis 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 ofiterator.

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 inPEP 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 awaitexpressions. Custom implementations must provide the __await__()method.

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

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(), andclose(). Custom implementations must also implement__await__(). All Coroutine instances are also instances ofAwaitable. See also the definition of coroutine.

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__() andisdisjoint():

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.

See also

• OrderedSet recipe for an example built on MutableSet.
• For more about ABCs, see the abc module and PEP 3119.