# 8.4. `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。

## 8.4.1. Collections Abstract Base Classes

The collections module offers the following ABCs:

ABC | Inherits from | Abstract Methods | Mixin Methods
--- | --- | --- | ---
`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` | Inherited `Sequence` methods and `append`, `reverse`, `extend`, `pop`, `remove`, and `__iadd__`
`ByteString` | `Sequence` | `__getitem__`, `__len__` | Inherited `Sequence` methods
`Set` | `Collection` | `__contains__`, `__iter__`, `__len__` | `__le__`, `__lt__`, `__eq__`, `__ne__`, `__gt__`, `__ge__`, `__and__`, `__or__`, `__sub__`, `__xor__`, and `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__`, and `__ne__`
`MutableMapping` | `Mapping` | `__getitem__`, `__setitem__`, `__delitem__`, `__iter__`, `__len__` | Inherited `Mapping` methods and `pop`, `popitem`, `clear`, `update`, and `setdefault`
`MappingView` | `Sized` |  | `__len__`
`ItemsView` | `MappingView`, `Set` |  | `__contains__`, `__iter__`
`KeysView` | `MappingView`, `Set` |  | `__contains__`, `__iter__`
`ValuesView` | `MappingView` |  | `__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`

  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.

  3.6 新版功能.

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.

  3.6 新版功能.

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.

  3.5 新版功能.

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.

  在 3.5 版更改: 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.

  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, Awaitable)` for them will return `False`. Use `inspect.isawaitable()` to detect them.

  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.