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

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.

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 thein-operator, the iterationlogic, 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:

Footnotes

Collections Abstract Base Classes – Detailed Descriptions¶

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.

See Annotating callable objects for details on how to useCallable in type annotations.

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.

Added 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.

Added in version 3.6.

class collections.abc.Generator¶

ABC for generator classes that implement the protocol defined inPEP 342 that extends iterators with thesend(),throw() and close() methods.

See Annotating generators and coroutinesfor details on using Generator in type annotations.

Added 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.

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, orcollections.abc.Buffer. For use as an ABC, prefer Sequence or collections.abc.Buffer.

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.

Added 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 of Awaitable.

See Annotating generators and coroutinesfor details on using Coroutine in type annotations. The variance and order of type parameters correspond to those ofGenerator.

Added in version 3.5.

class collections.abc.AsyncIterable¶

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

Added in version 3.5.

class collections.abc.AsyncIterator¶

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

Added in version 3.5.

class collections.abc.AsyncGenerator¶

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

See Annotating generators and coroutinesfor details on using AsyncGenerator in type annotations.

Added in version 3.6.

class collections.abc.Buffer¶

ABC for classes that provide the __buffer__() method, implementing the buffer protocol. See PEP 688.

Added in version 3.12.

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: __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 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 __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.