抽象基类Sequence的issubclass [英] issubclass of abstract base class Sequence

问题描述

此列表显示了您使用的方法需要实现将您的类视为"序列:__getitem__，__len__，__contains__，__iter__，__reversed__，index和count.那么，为什么这种最低限度的实现不起作用，即为什么issubclass(S, Sequence) is False?

This list shows what methods you need to implement for your class to be "regarded" as Sequence: __getitem__, __len__, __contains__, __iter__, __reversed__, index, and count. So why does this minimal implementation does not work, i.e. why issubclass(S, Sequence) is False?

from collections import *


class S(object):
    def __getitem__(self, item):
        raise IndexError

    def __len__(self):
        return 0

    def __contains__(self, item):
        return False

    def __iter__(self):
        return iter(())

    def __reversed__(self):
        return self

    def index(self, item):
        raise IndexError

    def count(self, item):
        return 0


issubclass(S, Iterable)   # True  :-)
issubclass(S, Sized)      # True  :-)
issubclass(S, Container)  # True  :-)
issubclass(S, Sequence)   # False :-(

是否有我需要实现的其他方法而被我忽略了?我是否误解了抽象基类?子类化Sequence当然会使issubclass返回True，但这有点使abc背后的想法失败了，不是吗?

Is there an additional method I need to implement that I overlooked? Did I misunderstand abstract base classes? Subclassing Sequence makes issubclass return True of course, but that kinda defeats the idea behind abc, doesn't it?

推荐答案

使用消息来源，卢克！

Sequence不会实现自己的__subclasshook__，并且所有来自Sequence父级的__subclasshook__实现都具有以下检查功能:

Sequence does not implement its own __subclasshook__, and all the implementations of __subclasshook__ from the parents of Sequence have checks like this:

class Iterable:
    ...

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Iterable:  # <<<<
            if _hasattr(C, "__iter__"):
                return True
        return NotImplemented

不过，您可以明确 register() 您的类作为Sequence:

Sequence.register(S)

关于Sequence不实现__subclasshook__的原因，请参见问题16728 (其中标题最初是"collections.abc.顺序应提供__subclasshook __" ).可以通过说一个序列来概括这个问题，这取决于谁使用它的需要:

As for the reason why Sequence does not implement __subclasshook__, see issue 16728 (which title was initially "collections.abc.Sequence shoud provide __subclasshook__"). The issue can be summarized by saying that a sequence can be many things, depending on the needs of who uses it:

许多需要序列的算法只需要__len__和__getitem__. [...] collections.abc.Sequence是一个更加丰富的界面.

Many algorithms that require a sequence only need __len__ and __getitem__. [...] collections.abc.Sequence is a much richer interface.

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