[issue5135] Expose simplegeneric function in functools module

[Éric Araujo]

Éric Araujo <merwok at netwok.org> added the comment:

> The register() method of an ABC only fakes out isinstance checks, it
> doesn't actually make the abstract base class a base class of the class.
> It doesn't make any sense for a class to say it is an instance of an
> ABC, but not have that ABC in [its] MRO.

I disagree.  If someone writes a class and registers them with an ABC, it is their duty to make sure that the class actually complies.  Virtual subclasses are provided for use by consenting adults IMO.

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nosy: +rhettinger

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<http://bugs.python.org/issue5135>
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