# `collections.abc` --- Abstract Base Classes for Containers

Added in version 3.3: 以前はこのモジュールは `collections` モジュールの一部でした。

ソースコード: 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.

`issubclass()` や `isinstance()` を使ったインターフェースに対するテストは、以下の3つのいずれかの方法で動作します。

1） 新しく定義したクラスは抽象基底クラスのいずれかを直接継承することができます。その場合クラスは必要な抽象メソッドを提供しなければなりません。残りのミックスインメソッドは継承により引き継がれますが、必要ならオーバーライドすることができます。その他のメソッドは必要に応じて追加することができます:

    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） 既存のクラスや組み込みのクラスを "仮想派生クラス" として ABC に登録することができます。これらのクラスは、全ての抽象メソッドとミックスインメソッドを含む完全な API を定義する必要があります。これにより、そのクラスが完全なインターフェースをサポートしているかどうかを、ユーザーが `issubclass()` や `isinstance()` で判断できるようになります。このルールの例外は、残りの API から自動的に推測ができるようなメソッドです:

    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） いくつかの単純なインターフェースは、必要なメソッドの存在だけで (それらのメソッドが `None` に設定されていなければ) 直接認識されます:

    class E:
        def __iter__(self): ...
        def __next__(self): ...

    >>> issubclass(E, Iterable)
    True
    >>> isinstance(E(), Iterable)
    True

複雑なインターフェースは、単に特定のメソッドが存在すること以上の定義を持つため、3番目のテクニックをサポートしていません。それらのインターフェースはメソッドの意味やメソッド間の関係まで指定するので、特定のメソッド名の存在からだけではインターフェースの推測ができません。たとえば、あるクラスが `__getitem__`, `__len__`, および `__iter__` を提供するというだけでは、 `Sequence` と `Mapping` を区別するには不十分です。

Added in version 3.9: これらの抽象クラスは `[]` をサポートするようになりました。 ジェネリックエイリアス型 および PEP 585 を参照してください。

## コレクション抽象基底クラス

collections モジュールは以下の ABC (抽象基底クラス) を提供します:

ABC | 継承しているクラス | 抽象メソッド | mixin メソッド
--- | --- | --- | ---
`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`, `Container` | `__contains__`, `__iter__`, `__len__` |
`Sequence` | `Reversible`, `Collection` | `__getitem__`, `__len__` | `__contains__`, `__iter__`, `__reversed__`, `index`, `count`
`MutableSequence` | `Sequence` | `__getitem__`, `__setitem__`, `__delitem__`, `__len__`, `insert` | Inherited `Sequence` methods and `append`, `clear`, `reverse`, `extend`, `pop`, `remove`, and `__iadd__`
`ByteString` | `Sequence` | `__getitem__`, `__len__` | `Sequence` から継承したメソッド
`Set` | `Collection` | `__contains__`, `__iter__`, `__len__` | `__le__`, `__lt__`, `__eq__`, `__ne__`, `__gt__`, `__ge__`, `__and__`, `__or__`, `__sub__`, `__xor__`, `isdisjoint`
`MutableSet` | `Set` | `__contains__`, `__iter__`, `__len__`, `add`, `discard` | `Set` から継承したメソッドと、 `clear`, `pop`, `remove`, `__ior__`, `__iand__`, `__ixor__`, `__isub__`
`Mapping` | `Collection` | `__getitem__`, `__iter__`, `__len__` | `__contains__`, `keys`, `items`, `values`, `get`, `__eq__`, `__ne__`
`MutableMapping` | `Mapping` | `__getitem__`, `__setitem__`, `__delitem__`, `__iter__`, `__len__` | `Mapping` から継承したメソッドと、 `pop`, `popitem`, `clear`, `update`, `setdefault`
`MappingView` | `Sized` |  | `__len__`
`ItemsView` | `MappingView`, `Set` |  | `__contains__`, `__iter__`
`KeysView` | `MappingView`, `Set` |  | `__contains__`, `__iter__`
`ValuesView` | `MappingView`, `Collection` |  | `__contains__`, `__iter__`
`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__`
`Buffer` [1] |  | `__buffer__` |

脚注

[1] (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15)

These ABCs override `__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)`.

## コレクションの抽象基底クラス -- 詳細な説明

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

  サイズ付きのイテラブルなコンテナクラスの ABC です。

  Added in version 3.6.

class collections.abc.Iterator

  `__iter__()` メソッドと `__next__()` メソッドを提供するクラスの ABC です。 iterator の定義も参照してください。

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.

  Added in version 3.5.

class collections.abc.Sequence
class collections.abc.MutableSequence
class collections.abc.ByteString

  読み出し専用の シーケンス およびミュータブルな シーケンス の ABC です。

  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.

  バージョン 3.5 で変更: index() メソッドは stop と start 引数をサポートしました。

  バージョン 3.12 で非推奨、バージョン 3.14 で削除予定: The `ByteString` ABC has been deprecated. For use in typing, prefer a union, like `bytes | bytearray`, or `collections.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

  読み出し専用の マッピング およびミュータブルな マッピング の ABC です。

class collections.abc.MappingView
class collections.abc.ItemsView
class collections.abc.KeysView
class collections.abc.ValuesView

  マッピング、要素、キー、値の ビュー の ABC です。

class collections.abc.Awaitable

  ABC for awaitable objects, which can be used in `await` expressions. Custom implementations must provide the `__await__()` method.

  `Coroutine` ABC の Coroutine オブジェクトとインスタンスは、すべてこの ABC のインスタンスです。

  注釈

  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 コルーチンオブジェクト: `send()`, `throw()`, and `close()`. Custom implementations must also implement `__await__()`. All `Coroutine` instances are also instances of `Awaitable`.

  注釈

  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.

  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

  `__aiter__` および `__anext__` メソッドを提供するクラスの ABC です。asynchronous iterator の定義も参照してください。

  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.

  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.

## 例とレシピ

抽象基底クラスは、クラスやインスタンスが特定の機能を提供しているかどうかを調べることを可能にします。例えば:

    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

`Set` と `MutableSet` を mixin 型として利用するときの注意点:

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`.

参考

- `MutableSet` を使った例として OrderedSet recipe。

- ABCs についての詳細は、 `abc` モジュールと PEP 3119 を参照してください。