collections.abc — Abstract Base Classes for Containers¶

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 use Callable 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 iterable is 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 of iterator.

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 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.

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.

index(value, start=0, stop=None)¶

Return first index of value.

Raises ValueError if the value is not present.

Supporting the start and stop arguments is optional, but recommended.

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

Deprecated since version 3.12, will be removed in version 3.17: The ByteString ABC has been deprecated.

Use isinstance(obj, collections.abc.Buffer) to test if obj implements the buffer protocol at runtime. For use in type annotations, either use Buffer or a union that explicitly specifies the types your code supports (e.g., bytes | bytearray | memoryview).

ByteString was originally intended to be an abstract class that would serve as a supertype of both bytes and bytearray. However, since the ABC never had any methods, knowing that an object was an instance of ByteString never actually told you anything useful about the object. Other common buffer types such as memoryview were also never understood as subtypes of ByteString (either at runtime or by static type checkers).

See PEP 688 for more details.

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) 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.

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(), and close(). Custom implementations must also implement __await__(). All Coroutine instances are also instances of Awaitable.

Note

In CPython, generator-based coroutines (generators decorated with @types.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.

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.

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 coroutines for 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.