[Python-3000] ABC PEP isinstance issue Was: PEP 31XX: A Type Hierarchy for Numbers (and other algebraic entities)

[Phillip J. Eby]

At 06:40 PM 4/25/2007 -0400, Jean-Paul Calderone wrote:
>On Wed, 25 Apr 2007 18:10:23 -0400, Jim Jewett <jimjjewett at gmail.com> wrote:
> >The current ABC proposal is to use isinstance as the test; Jeffrey
> >Yaskin's numbers PEP highlighted the weakness there with a concrete
> >example.
> >
> >If you need to an abstraction less powerful than an existing ABC,
> >you're out of luck; you can't just assert that the existing class is
> >already sufficient, nor can you expect everyone else to use multiple
> >annotations.
>
>I'm sure everyone is already aware of the behavior of the classImplements
>and directlyProvides functions available in zope.interface, which exactly
>satisfy this use-case in the interface world.

I'm either misunderstanding Jim or you, because I don't see the 
relationship here.  If I understand Jim correctly, he's actually asking for 
something like PyProtocols' "protocolIsSubsetOf" declaration -- something 
that would be like dynamically adding a superclass to an existing interface 
in zope.interface.  Whereas the features you're talking about sound like 
declaring that a class object itself implements an interface -- something 
apparently unrelated to the question at hand.