[Python-3000] ABC's, Roles, etc

[Greg Ewing]

Guido van Rossum wrote:

> From this point, issubclass(InstrumentedList, Sequence) will be true
> (and likewise for instances of it and isinstance(x, Sequence)). But
> InstrumentedList's __mro__ and __bases__ are unchanged.

I think I've figured out what bothers me about this
kind of overloading of isinstance(). Normally if
isinstance(x, C) is true, we expect that a method
call on x can at least potentially invoke a method
of class C. But if isinstance(x, C) can be true
even if C doesn't appear in the mro of x, this is
no longer the case.

This isn't so much of a worry when x is acting as a
proxy for C, since it's probably forwarding method
calls to a real instance of C somewhere. But using
it in a more general way seems strange.

--
Greg