[Python-3000] Revised PEP 3119 (Abstract Base Classes)

[Samuele Pedroni]

Guido van Rossum wrote:
> **Open issues:** Conceivably, instead of using the ABCMeta metaclass,
> these classes could override ``__instancecheck__`` and
> ``__subclasscheck__`` to check for the presence of the applicable
> special method; for example::
>
>     class Sized(metaclass=ABCMeta):
>         @abstractmethod
>         def __hash__(self):
>             return 0
>         @classmethod
>         def __instancecheck__(cls, x):
>             return hasattr(x, "__len__")
>         @classmethod
>         def __subclasscheck__(cls, C):
>             return hasattr(C, "__bases__") and hasattr(C, "__len__")
>
> This has the advantage of not requiring explicit registration.
> However, the semantics hard to get exactly right given the confusing
> semantics of instance attributes vs. class attributes, and that a
> class is an instance of its metaclass; the check for ``__bases__`` is
> only an approximation of the desired semantics.  **Strawman:** Let's
> do it, but let's arrange it in such a way that the registration API
> also works.
>
>
>   
just to say that I still think the strawman would be right thing to do.