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

[Jean-Paul Calderone]

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.

Jean-Paul