`!collections.abc` --- Abstract Base Classes for Containers

Source code: Lib/_collections_abc.py

This module provides abstract base classes <abstract base class> 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.

1. 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

2. Existing classes and built-in classes can be registered as "virtual subclasses" 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 <comparisons>, the iteration <iterable> logic, and the reversed function automatically fall back to using __getitem__ and __len__.

3. 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.

Collections Abstract Base Classes

The collections module offers the following ABCs <abstract base class>:

Footnotes

Collections Abstract Base Classes -- Detailed Descriptions

Examples and Recipes

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: ~object.__contains__, ~container.__iter__, and ~object.__len__. The ABC supplies the remaining methods such as !__and__ and ~frozenset.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 or regular method that can construct new instances from an iterable argument.

(2)
To override the comparisons (presumably for speed, as the semantics are fixed), redefine ~object.__le__ and ~object.__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, ~object.__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.

[^1]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^2]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^3]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^4]: Checking isinstance(obj, Iterable) detects classes that are registered as Iterable or that have an ~container.__iter__ method, but it does not detect classes that iterate with the ~object.__getitem__ method. The only reliable way to determine whether an object is iterable is to call iter(obj).

[^5]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^6]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^7]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^8]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^9]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^10]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^11]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^12]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^13]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^14]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^15]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.

[^16]: These ABCs override ~abc.ABCMeta.__subclasshook__ to support testing an interface by verifying the required methods are present and have not been set to None. This only works for simple interfaces. More complex interfaces require registration or direct subclassing.