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Python 3.6.5 Documentation > "collections.abc" — Abstract Base Classes for Containers

"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.

Collections Abstract Base Classes
=================================

The collections module offers the following *ABCs*:

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.

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.

New 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.

New 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 also the definition of *generator*.

New 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.

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()" or "asyncio.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.

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()", and "close()". Custom implementations must also
implement "__await__()". All "Coroutine" instances are also
instances of "Awaitable". See also the definition of *coroutine*.

Note: In CPython, generator-based coroutines (generators
decorated with "types.coroutine()" or "asyncio.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.

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.

class collections.abc.AsyncGenerator

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

New in version 3.6.

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:

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 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".