# `collections.abc` --- 容器的抽象基底類別

在 3.3 版被加入: 過去此模組是 `collections` 模組的一部分。

原始碼：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__(self): ...

       >>> 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`.

在 3.9 版被加入: These abstract classes now support `[]`. See 泛型別名型別 and PEP 585.

## Collections Abstract Base Classes

The collections module offers the following ABCs:

ABC | Inherits from | 抽象方法 | 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`、`Container` | `__contains__`、`__iter__`、`__len__` |
`Sequence` | `Reversible`, `Collection` | `__getitem__`、`__len__` | `__contains__`、`__iter__`、`__reversed__`、`index` 和 `count`
`MutableSequence` | `Sequence` | `__getitem__`、`__setitem__`、`__delitem__`、`__len__`、`insert` | 繼承 `Sequence` 方法和 `append`、`clear`、`reverse`、`extend`、`pop`、`remove` 和 `__iadd__`
`ByteString` | `Sequence` | `__getitem__`、`__len__` | `Sequence` 的繼承方法
`Set` | `Collection` | `__contains__`、`__iter__`、`__len__` | `__le__`、`__lt__`、`__eq__`、`__ne__`、`__gt__`、`__ge__`、`__and__`、`__or__`、`__sub__`、`__rsub__`、`__xor__`、`__rxor__` 和 `isdisjoint`
`MutableSet` | `Set` | `__contains__`、`__iter__`、`__len__`、`add`、`discard` | Inherited `Set` methods and `clear`, `pop`, `remove`, `__ior__`, `__iand__`, `__ixor__`, and `__isub__`
`Mapping` | `Collection` | `__getitem__`、`__iter__`、`__len__` | `__contains__`、`keys`、`items`、`values`、`get`、`__eq__` 和 `__ne__`
`MutableMapping` | `Mapping` | `__getitem__`、`__setitem__`、`__delitem__`、`__iter__`、`__len__` | Inherited `Mapping` methods and `pop`, `popitem`, `clear`, `update`, and `setdefault`
`MappingView` | `Sized` |  | `__init__`、`__len__` 和 `__repr__`
`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__` |

註腳

[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)`.

## 集合抽象基底類別的詳細描述

class collections.abc.Container

  有提供 `__contains__()` 方法之類別的 ABC。

class collections.abc.Hashable

  有提供 `__hash__()` 方法之類別的 ABC。

class collections.abc.Sized

  有提供 `__len__()` 方法之類別的 ABC。

class collections.abc.Callable

  有提供 `__call__()` 方法之類別的 ABC。

  See 註釋 callable 物件 for details on how to use `Callable` in type annotations.

class collections.abc.Iterable

  有提供 `__iter__()` 方法之類別的 ABC。

  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.

  在 3.6 版被加入.

class collections.abc.Iterator

  有提供 `__iter__()` 和 `__next__()` 方法之類別的 ABC。另請參閱 iterator 的定義。

class collections.abc.Reversible

  亦有提供 `__reversed__()` 方法之可疊代類別的 ABC。

  在 3.6 版被加入.

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.

  在 3.5 版被加入.

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

  唯讀且可變序列的 ABC。

  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)

    回傳 value 的第一個索引。

    如果找不到該值，則引發 `ValueError`。

    建議要支援 start 和 stop 引數，但並非必要。

    在 3.5 版的變更: The `index()` method gained support for the stop and start arguments.

  自從版本 3.12 後不推薦使用，將會自版本 3.17 中移除。: `ByteString` ABC 已被棄用。

  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` 最初被設計為一個抽象類別，以作為 `bytes` 和 `bytearray` 的超型別 (supertype)。然而由於 ABC 從未擁有任何方法，知道一個物件是 `ByteString` 的實例從未真正告訴你任何關於該物件的有用資訊。其他常見的緩衝區型別如 `memoryview` 也從未被理解為 `ByteString` 的子型別（無論是在 runtime 還是由靜態型別檢查器）。

  更多詳細資訊請參閱 PEP 688。

class collections.abc.Set
class collections.abc.MutableSet

  唯讀且可變集合的 ABC。

class collections.abc.Mapping
class collections.abc.MutableMapping

  唯讀且可變對映的 ABC。

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.

  備註

  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.

  在 3.5 版被加入.

class collections.abc.Coroutine

  ABC for coroutine compatible classes. These implement the following methods, defined in 協程物件: `send()`, `throw()`, and `close()`. Custom implementations must also implement `__await__()`. All `Coroutine` instances are also instances of `Awaitable`.

  備註

  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`.

  在 3.5 版被加入.

class collections.abc.AsyncIterable

  ABC for classes that provide an `__aiter__` method. See also the definition of asynchronous iterable.

  在 3.5 版被加入.

class collections.abc.AsyncIterator

  ABC for classes that provide `__aiter__` and `__anext__` methods. See also the definition of asynchronous iterator.

  在 3.5 版被加入.

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.

  在 3.6 版被加入.

class collections.abc.Buffer

  ABC for classes that provide the `__buffer__()` method, implementing the buffer protocol. See PEP 688.

  在 3.12 版被加入.

## 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`.

也參考

- OrderedSet recipe for an example built on `MutableSet`.

- 關於 ABC 的更多資訊請見 `abc` module 和 PEP 3119。