collections.abc â Abstract Base Classes for Containers¶
Added 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.
An issubclass() or isinstance() test for an interface works in one of three ways.
A newly written class can inherit directly from one of the abstract base classes. The class must supply the required abstract methods. The remaining mixin methods come from inheritance and can be overridden if desired. Other methods may be added as needed:
class C(Sequence): # Direct inheritance
def __init__(self): ... # Extra method not required by the ABC
def __getitem__(self, index): ... # Required abstract method
def __len__(self): ... # Required abstract method
def count(self, value): ... # Optionally override a mixin method
>>> issubclass(C, Sequence) True >>> isinstance(C(), Sequence) True
Existing classes and built-in classes can be registered as âvirtual subclassesâ of the ABCs. Those classes should define the full API including all of the abstract methods and all of the mixin methods. This lets users rely on issubclass() or isinstance() tests to determine whether the full interface is supported. The exception to this rule is for methods that are automatically inferred from the rest of the API:
class D: # No inheritance
def __init__(self): ... # Extra method not required by the ABC
def __getitem__(self, index): ... # Abstract method
def __len__(self): ... # Abstract method
def count(self, value): ... # Mixin method
def index(self, value): ... # Mixin method
Sequence.register(D) # Register instead of inherit
>>> issubclass(D, Sequence) True >>> isinstance(D(), Sequence) True
In this example, class D does not need to define __contains__, __iter__, and __reversed__ because the in-operator, the iteration logic, and the reversed() function automatically fall back to using __getitem__ and __len__.
Some simple interfaces are directly recognizable by the presence of the required methods (unless those methods have been set to None):
class E:
def __iter__(self): ...
def __next__(self): ...
>>> issubclass(E, Iterable) True >>> isinstance(E(), Iterable) True
Complex interfaces do not support this last technique because an interface is more than just the presence of method names. Interfaces specify semantics and relationships between methods that cannot be inferred solely from the presence of specific method names. For example, knowing that a class supplies __getitem__, __len__, and __iter__ is insufficient for distinguishing a Sequence from a Mapping.
Added in version 3.9: These abstract classes now support []. See Generic Alias Type and PEP 585.
The collections module offers the following ABCs:
ABC
Inherits from
Abstract Methods
Mixin Methods
__contains__
__hash__
__iter__
__next__
__iter__
__reversed__
send, throw
close, __iter__, __next__
__len__
__call__
__contains__, __iter__, __len__
__getitem__, __len__
__contains__, __iter__, __reversed__, index, and count
__getitem__, __setitem__, __delitem__, __len__, insert
Inherited Sequence methods and append, clear, reverse, extend, pop, remove, and __iadd__
__getitem__, __len__
Inherited Sequence methods
__contains__, __iter__, __len__
__le__, __lt__, __eq__, __ne__, __gt__, __ge__, __and__, __or__, __sub__, __rsub__, __xor__, __rxor__ and isdisjoint
__contains__, __iter__, __len__, add, discard
Inherited Set methods and clear, pop, remove, __ior__, __iand__, __ixor__, and __isub__
__getitem__, __iter__, __len__
__contains__, keys, items, values, get, __eq__, and __ne__
__getitem__, __setitem__, __delitem__, __iter__, __len__
Inherited Mapping methods and pop, popitem, clear, update, and setdefault
__init__, __len__ and __repr__
__contains__, __iter__
__contains__, __iter__
__contains__, __iter__
__await__
send, throw
close
__aiter__
__anext__
__aiter__
asend, athrow
aclose, __aiter__, __anext__
__buffer__
Footnotes
Collections Abstract Base Classes â Detailed Descriptions¶ABC for classes that provide the __contains__() method.
ABC for classes that provide the __hash__() method.
ABC for classes that provide the __len__() method.
ABC for classes that provide the __call__() method.
See Annotating callable objects for details on how to use Callable in type annotations.
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).
ABC for sized iterable container classes.
Added in version 3.6.
ABC for classes that provide the __iter__() and __next__() methods. See also the definition of iterator.
ABC for iterable classes that also provide the __reversed__() method.
Added in version 3.6.
ABC for generator classes that implement the protocol defined in PEP 342 that extends iterators with the send(), throw() and close() methods.
See Annotating generators and coroutines for details on using Generator in type annotations.
Added in version 3.5.
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.
Deprecated since version 3.12, will be removed in version 3.14: The ByteString ABC has been deprecated. For use in typing, prefer a union, like bytes | bytearray, or collections.abc.Buffer. For use as an ABC, prefer Sequence or collections.abc.Buffer.
ABCs for read-only and mutable sets.
ABCs for read-only and mutable mappings.
ABCs for mapping, items, keys, and values views.
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.
Added in version 3.5.
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 Annotating generators and coroutines for details on using Coroutine in type annotations. The variance and order of type parameters correspond to those of Generator.
Added in version 3.5.
ABC for classes that provide an __aiter__ method. See also the definition of asynchronous iterable.
Added in version 3.5.
ABC for classes that provide __aiter__ and __anext__ methods. See also the definition of asynchronous iterator.
Added in version 3.5.
ABC for asynchronous generator classes that implement the protocol defined in PEP 525 and PEP 492.
See Annotating generators and coroutines for details on using AsyncGenerator in type annotations.
Added in version 3.6.
ABC for classes that provide the __buffer__() method, implementing the buffer protocol. See PEP 688.
Added in version 3.12.
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:
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 or regular method that can construct new instances from an iterable argument.
To override the comparisons (presumably for speed, as the semantics are fixed), redefine __le__() and __ge__(), then the other operations will automatically follow suit.
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.
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