8.4. collections.abc
— Abstract Base Classes for Containers
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.
8.4.1. Collections Abstract Base Classes
The collections module offers the following ABCs:
ABC | Inherits from | Abstract Methods | Mixin Methods |
---|---|---|---|
Container |
__contains__ |
||
Hashable |
__hash__ |
||
Iterable |
__iter__ |
||
Iterator |
Iterable |
__next__ |
__iter__ |
Generator |
Iterator |
send , throw |
close , __iter__ , __next__ |
Sized |
__len__ |
||
Callable |
__call__ |
||
Sequence |
Sized ,
Iterable ,
Container |
__getitem__ ,
__len__ |
__contains__ , __iter__ , __reversed__ ,
index , and count |
MutableSequence |
Sequence |
__getitem__ ,
__setitem__ ,
__delitem__ ,
__len__ ,
insert |
Inherited Sequence methods and
append , reverse , extend , pop ,
remove , and __iadd__ |
Set |
Sized ,
Iterable ,
Container |
__contains__ ,
__iter__ ,
__len__ |
__le__ , __lt__ , __eq__ , __ne__ ,
__gt__ , __ge__ , __and__ , __or__ ,
__sub__ , __xor__ , and isdisjoint |
MutableSet |
Set |
__contains__ ,
__iter__ ,
__len__ ,
add ,
discard |
Inherited Set methods and
clear , pop , remove , __ior__ ,
__iand__ , __ixor__ , and __isub__ |
Mapping |
Sized ,
Iterable ,
Container |
__getitem__ ,
__iter__ ,
__len__ |
__contains__ , keys , items , values ,
get , __eq__ , and __ne__ |
MutableMapping |
Mapping |
__getitem__ ,
__setitem__ ,
__delitem__ ,
__iter__ ,
__len__ |
Inherited Mapping methods and
pop , popitem , clear , update ,
and setdefault |
MappingView |
Sized |
__len__ |
|
ItemsView |
MappingView ,
Set |
__contains__ ,
__iter__ |
|
KeysView |
MappingView ,
Set |
__contains__ ,
__iter__ |
|
ValuesView |
MappingView |
__contains__ , __iter__ |
|
Awaitable |
__await__ |
||
Coroutine |
Awaitable |
send , throw |
close |
AsyncIterable |
__aiter__ |
||
AsyncIterator |
AsyncIterable |
__anext__ |
__aiter__ |
-
class
collections.abc.
Container
-
class
collections.abc.
Hashable
-
class
collections.abc.
Sized
-
class
collections.abc.
Callable
ABCs for classes that provide respectively the methods
__contains__()
,__hash__()
,__len__()
, and__call__()
.
-
class
collections.abc.
Iterable
ABC for classes that provide the
__iter__()
method. See also the definition of iterable.
-
class
collections.abc.
Iterator
ABC for classes that provide the
__iter__()
and__next__()
methods. See also the definition of iterator.
-
class
collections.abc.
Generator
ABC for generator classes that implement the protocol defined in PEP 342 that extends iterators with the
send()
,throw()
andclose()
methods. See also the definition of generator.New in version 3.5.
-
class
collections.abc.
Sequence
-
class
collections.abc.
MutableSequence
ABCs for read-only and mutable sequences.
Implementation note: Some of the mixin methods, such as
__iter__()
,__reversed__()
andindex()
, 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
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()
orasyncio.coroutine()
) are awaitables, even though they do not have an__await__()
method. Usingisinstance(gencoro, Awaitable)
for them will returnFalse
. Useinspect.isawaitable()
to detect them.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__()
. AllCoroutine
instances are also instances ofAwaitable
. See also the definition of coroutine.Note
In CPython, generator-based coroutines (generators decorated with
types.coroutine()
orasyncio.coroutine()
) are awaitables, even though they do not have an__await__()
method. Usingisinstance(gencoro, Coroutine)
for them will returnFalse
. Useinspect.isawaitable()
to detect them.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.
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__()
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 callscls(iterable)
to produce a new set. If theSet
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. - 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 bothSet()
andHashable()
, 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.