"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.

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, the
*iteration* 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__(next): ...

   >>> 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".

New in version 3.9: These abstract classes now support "[]". See
Generic Alias Type and **PEP 585**.


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

The collections module offers the following *ABCs*:

+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| ABC                            | Inherits from          | Abstract Methods        | Mixin Methods                                        |
|================================|========================|=========================|======================================================|
| "Container" [1]                |                        | "__contains__"          |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Hashable" [1]                 |                        | "__hash__"              |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Iterable" [1] [2]             |                        | "__iter__"              |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Iterator" [1]                 | "Iterable"             | "__next__"              | "__iter__"                                           |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Reversible" [1]               | "Iterable"             | "__reversed__"          |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Generator"  [1]               | "Iterator"             | "send", "throw"         | "close", "__iter__", "__next__"                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Sized"  [1]                   |                        | "__len__"               |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Callable"  [1]                |                        | "__call__"              |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Collection"  [1]              | "Sized", "Iterable",   | "__contains__",         |                                                      |
|                                | "Container"            | "__iter__", "__len__"   |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Sequence"                     | "Reversible",          | "__getitem__",          | "__contains__", "__iter__", "__reversed__", "index", |
|                                | "Collection"           | "__len__"               | and "count"                                          |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "MutableSequence"              | "Sequence"             | "__getitem__",          | Inherited "Sequence" methods and "append",           |
|                                |                        | "__setitem__",          | "reverse", "extend", "pop", "remove", and "__iadd__" |
|                                |                        | "__delitem__",          |                                                      |
|                                |                        | "__len__", "insert"     |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "ByteString"                   | "Sequence"             | "__getitem__",          | Inherited "Sequence" methods                         |
|                                |                        | "__len__"               |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Set"                          | "Collection"           | "__contains__",         | "__le__", "__lt__", "__eq__", "__ne__", "__gt__",    |
|                                |                        | "__iter__", "__len__"   | "__ge__", "__and__", "__or__", "__sub__", "__xor__", |
|                                |                        |                         | and "isdisjoint"                                     |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "MutableSet"                   | "Set"                  | "__contains__",         | Inherited "Set" methods and "clear", "pop",          |
|                                |                        | "__iter__", "__len__",  | "remove", "__ior__", "__iand__", "__ixor__", and     |
|                                |                        | "add", "discard"        | "__isub__"                                           |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Mapping"                      | "Collection"           | "__getitem__",          | "__contains__", "keys", "items", "values", "get",    |
|                                |                        | "__iter__", "__len__"   | "__eq__", and "__ne__"                               |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "MutableMapping"               | "Mapping"              | "__getitem__",          | Inherited "Mapping" methods and "pop", "popitem",    |
|                                |                        | "__setitem__",          | "clear", "update", and "setdefault"                  |
|                                |                        | "__delitem__",          |                                                      |
|                                |                        | "__iter__", "__len__"   |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "MappingView"                  | "Sized"                |                         | "__len__"                                            |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "ItemsView"                    | "MappingView", "Set"   |                         | "__contains__", "__iter__"                           |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "KeysView"                     | "MappingView", "Set"   |                         | "__contains__", "__iter__"                           |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "ValuesView"                   | "MappingView",         |                         | "__contains__", "__iter__"                           |
|                                | "Collection"           |                         |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Awaitable" [1]                |                        | "__await__"             |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "Coroutine" [1]                | "Awaitable"            | "send", "throw"         | "close"                                              |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "AsyncIterable" [1]            |                        | "__aiter__"             |                                                      |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "AsyncIterator" [1]            | "AsyncIterable"        | "__anext__"             | "__aiter__"                                          |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+
| "AsyncGenerator" [1]           | "AsyncIterator"        | "asend", "athrow"       | "aclose", "__aiter__", "__anext__"                   |
+--------------------------------+------------------------+-------------------------+------------------------------------------------------+

-[ Footnotes ]-

[1] These ABCs override "object.__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] 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)".


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.

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.


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**.
