# `collections.abc` --- 容器的抽象基类

3.3 新版功能: 该模块曾是 `collections` 模块的组成部分。

源代码： Lib/_collections_abc.py

该模块定义了一些 抽象基类，它们可用于判断一个具体类是否具有某一特定的接口；例如，这个类是否可哈希，或其是否为映射类。

## 容器抽象基类

这个容器模块提供了以下 ABCs:

ABC | 继承自 | 抽象方法 | Mixin 方法
--- | --- | --- | ---
`Container` |  | `__contains__` |
`Hashable` |  | `__hash__` |
`Iterable` |  | `__iter__` |
`Iterator` | `Iterable` | `__next__` | `__iter__`
`Reversible` | `Iterable` | `__reversed__` |
`Generator` | `Iterator` | `send`, `throw` | `close`, `__iter__`, `__next__`
`Sized` |  | `__len__` |
`Callable` |  | `__call__` |
`Collection` | `Sized`, `Iterable`, `Container` | `__contains__`, `__iter__`, `__len__` |
`Sequence` | `Reversible`, `Collection` | `__getitem__`, `__len__` | `__contains__`, `__iter__`, `__reversed__`, `index`, and `count`
`MutableSequence` | `Sequence` | `__getitem__`, `__setitem__`, `__delitem__`, `__len__`, `insert` | 继承自 `Sequence` 的方法，以及 `append`, `reverse`, `extend`, `pop`, `remove`，和 `__iadd__`
`ByteString` | `Sequence` | `__getitem__`, `__len__` | 继承自 `Sequence` 的方法
`Set` | `Collection` | `__contains__`, `__iter__`, `__len__` | `__le__`, `__lt__`, `__eq__`, `__ne__`, `__gt__`, `__ge__`, `__and__`, `__or__`, `__sub__`, `__xor__`, and `isdisjoint`
`MutableSet` | `Set` | `__contains__`, `__iter__`, `__len__`, `add`, `discard` | 继承自 `Set` 的方法以及 `clear`, `pop`, `remove`, `__ior__`, `__iand__`, `__ixor__`，和 `__isub__`
`Mapping` | `Collection` | `__getitem__`, `__iter__`, `__len__` | `__contains__`, `keys`, `items`, `values`, `get`, `__eq__`, and `__ne__`
`MutableMapping` | `Mapping` | `__getitem__`, `__setitem__`, `__delitem__`, `__iter__`, `__len__` | 继承自 `Mapping` 的方法以及 `pop`, `popitem`, `clear`, `update`，和 `setdefault`
`MappingView` | `Sized` |  | `__len__`
`ItemsView` | `MappingView`, `Set` |  | `__contains__`, `__iter__`
`KeysView` | `MappingView`, `Set` |  | `__contains__`, `__iter__`
`ValuesView` | `MappingView`, `Collection` |  | `__contains__`, `__iter__`
`Awaitable` |  | `__await__` |
`Coroutine` | `Awaitable` | `send`, `throw` | `close`
`AsyncIterable` |  | `__aiter__` |
`AsyncIterator` | `AsyncIterable` | `__anext__` | `__aiter__`
`AsyncGenerator` | `AsyncIterator` | `asend`, `athrow` | `aclose`, `__aiter__`, `__anext__`

class `collections.abc.``Container`
class `collections.abc.``Hashable`
class `collections.abc.``Sized`
class `collections.abc.``Callable`

  分别提供了 `__contains__()`, `__hash__()`, `__len__()` 和 `__call__()` 方法的抽象基类。

class `collections.abc.``Iterable`

  提供了 `__iter__()` 方法的抽象基类。

  使用 `isinstance(obj, Iterable)` 可以检测一个类是否已经注册到了 `Iterable` 或者实现了 `__iter__()` 函数，但是无法检测这个类是否能够使用 `__getitem__()` 方法进行迭代。检测一个对象是否是 iterable 的唯一可信赖的方法是调用 `iter(obj)`。

class `collections.abc.``Collection`

  集合了 Sized 和 Iterable 类的抽象基类。

  3.6 新版功能.

class `collections.abc.``Iterator`

  提供了 `__iter__()` 和 `__next__()` 方法的抽象基类。参见 iterator 的定义。

class `collections.abc.``Reversible`

  为可迭代类提供了 `__reversed__()` 方法的抽象基类。

  3.6 新版功能.

class `collections.abc.``Generator`

  生成器类，实现了 PEP 342 中定义的协议，继承并扩展了迭代器，提供了 `send()`, `throw()` 和 `close()` 方法。参见 generator 的定义。

  3.5 新版功能.

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

  只读且可变的序列 sequences 的抽象基类。

  实现笔记：一些混入（Maxin）方法比如 `__iter__()`, `__reversed__()` 和 `index()` 会重复调用底层的 `__getitem__()` 方法。因此，如果实现的 `__getitem__()` 是常数级访问速度，那么相应的混入方法会有一个线性的表现；然而，如果底层方法是线性实现（例如链表），那么混入方法将会是平方级的表现，这也许就需要被重构了。

  在 3.5 版更改: index() 方法支持 stop 和 start 参数。

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

  只读且可变的集合的抽象基类。

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

  只读且可变的映射 mappings 的抽象基类。

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

  映射及其键和值的视图 views 的抽象基类。

class `collections.abc.``Awaitable`

  为可等待对象 awaitable 提供的类，可以被用于 `await` 表达式中。习惯上必须实现 `__await__()` 方法。

  协程对象 Coroutine 和 `Coroutine` 抽象基类的实例都是这个抽象基类的实例。

  注解

  在 CPython 里，基于生成器的协程（使用 `types.coroutine()` 或 `asyncio.coroutine()` 包装的生成器）都是 可等待对象，即使他们不含有 `__await__()` 方法。使用 `isinstance(gencoro, Awaitable)` 来检测他们会返回 `False`。要使用 `inspect.isawaitable()` 来检测他们。

  3.5 新版功能.

class `collections.abc.``Coroutine`

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

  注解

  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.

  3.5 新版功能.

class `collections.abc.``AsyncIterable`

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

  3.5 新版功能.

class `collections.abc.``AsyncIterator`

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

  3.5 新版功能.

class `collections.abc.``AsyncGenerator`

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

  3.6 新版功能.

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`.

参见

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

- For more about ABCs, see the `abc` module and PEP 3119.