Python 中，collections.abc 模塊下有很多和容器相關(guān)的一些數(shù)據(jù)結(jié)構(gòu)的抽象基類。

__all__ 中定義了和 collection 相關(guān)的基類，屬于內(nèi)置的抽象基類。

# Copyright 2007 Google, Inc. All Rights Reserved.
# Licensed to PSF under a Contributor Agreement.

"""Abstract Base Classes (ABCs) for collections, according to PEP 3119.

Unit tests are in test_collections.
"""

from abc import ABCMeta, abstractmethod
import sys

__all__ = ["Awaitable", "Coroutine",
           "AsyncIterable", "AsyncIterator", "AsyncGenerator",
           "Hashable", "Iterable", "Iterator", "Generator", "Reversible",
           "Sized", "Container", "Callable", "Collection",
           "Set", "MutableSet",
           "Mapping", "MutableMapping",
           "MappingView", "KeysView", "ItemsView", "ValuesView",
           "Sequence", "MutableSequence",
           "ByteString",
           ]

# This module has been renamed from collections.abc to _collections_abc to
# speed up interpreter startup. Some of the types such as MutableMapping are
# required early but collections module imports a lot of other modules.
# See issue #19218
__name__ = "collections.abc"

在這里，我們主要關(guān)注 "Sequence" 和 "MutableSequence"，分別為不可變的序列類型和不可變的序列類型。

先來看看 "Sequence"，其繼承了 Reversible, Collection 基類

### SEQUENCES ###
class Sequence(Reversible, Collection):

    """All the operations on a read-only sequence.

    Concrete subclasses must override __new__ or __init__,
    __getitem__, and __len__.
    """

    __slots__ = ()

    @abstractmethod
    def __getitem__(self, index):
        raise IndexError

    def __iter__(self):
        i = 0
        try:
            while True:
                v = self[i]
                yield v
                i += 1
        except IndexError:
            return

    def __contains__(self, value):
        for v in self:
            if v is value or v == value:
                return True
        return False

    def __reversed__(self):
        for i in reversed(range(len(self))):
            yield self[i]

    def index(self, value, start=0, stop=None):
        '''S.index(value, [start, [stop]]) -> integer -- return first index of value.
           Raises ValueError if the value is not present.
        '''
        if start is not None and start < 0:
            start = max(len(self) + start, 0)
        if stop is not None and stop < 0:
            stop += len(self)

        i = start
        while stop is None or i < stop:
            try:
                v = self[i]
                if v is value or v == value:
                    return i
            except IndexError:
                break
            i += 1
        raise ValueError

    def count(self, value):
        'S.count(value) -> integer -- return number of occurrences of value'
        return sum(1 for v in self if v is value or v == value)

Sequence.register(tuple)
Sequence.register(str)
Sequence.register(range)
Sequence.register(memoryview)

這是 Collection 基類的定義

class Collection(Sized, Iterable, Container):

    __slots__ = ()

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Collection:
            return _check_methods(C,  "__len__", "__iter__", "__contains__")
        return NotImplemented

class Reversible(Iterable):

    __slots__ = ()

    @abstractmethod
    def __reversed__(self):
        while False:
            yield None

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Reversible:
            return _check_methods(C, "__reversed__", "__iter__")
        return NotImplemented

Collection 繼承了 Sized, Iterable, Container 三個(gè)基類

class Sized(metaclass=ABCMeta):

    __slots__ = ()

    # 實(shí)現(xiàn)了 __len__ 方法，可以獲取其長度
    @abstractmethod
    def __len__(self):
        return 0

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Sized:
            return _check_methods(C, "__len__")
        return NotImplemented

class Iterable(metaclass=ABCMeta):

    __slots__ = ()

    # 實(shí)現(xiàn)了__iter__方法，可以進(jìn)行 for 循環(huán)
    @abstractmethod
    def __iter__(self):
        while False:
            yield None

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Iterable:
            return _check_methods(C, "__iter__")
        return NotImplemented


class Container(metaclass=ABCMeta):

    __slots__ = ()
    
    # 實(shí)現(xiàn)了 __contains__，可以使用 if-in 模式判斷。
    # Python 解釋器首先使用 __contains__
    # 沒有的話退一步找 __getitem__，然后遍歷整個(gè)序列
    @abstractmethod
    def __contains__(self, x):
        return False

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Container:
            return _check_methods(C, "__contains__")
        return NotImplemented

所以，在我們自己定義序列類型的時(shí)候，需要遵循這些協(xié)議。