[issue27598] Add SizedIterable to collections.abc and typing

[Neil Girdhar]

Neil Girdhar added the comment:

@gvanrossum is there any reason that subclasshook is implemented by overriding instead of cooperation?  E.g.,:

class Sized(metaclass=ABCMeta):

    @classmethod
    def __subclasshook__(cls, C):
        return (super().__subclasshook__(C) and 
                any("__len__" in B.__dict__ for B in C.__mro__))


etc.  And then Collection does not need to implement subclasshook since its base classes cooperate?

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nosy: +neil.g

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<http://bugs.python.org/issue27598>
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