[Python-3000] Abilities / Interfaces

[Bill Janssen]

> I'm concerned about the additional wrinkle where there is a library
> that calls the factory to produce A's and then turns around and passes
> them on to *another* library. Now the other can deal with A's but has
> recently been upgraded so that it works much better with B's; but it
> doesn't know that A's are fine B's already. ideally, there'd be an
> indirection in there somewhere that would let us insert a conversion
> from A to B; but if there isn't, it would be nice to be able to poke
> into A a marker indicating that it also supports B.

There's a dangling "it" in there which I'm not sure I have the right
antecedent for.  Let's see if I've got this straight :-).

     class B (A):
        ...

     def library_1_code():
        ...
        value_A = factory_which_really_produces_Bs()
        library_2_which_works_much_better_with_Bs(value_A)
        ...

Isn't the marker already implicit?  Does it matter that the first
library only thinks of the A interface for the value?  Or are you just
looking for a new built-in like

   def markas (i, interface_class):
     if not isinstance(i, interface_class):
         class X (i.__class__, interface_class):
             pass
         i.__class__ = X
         interface_class.__init__(i)
     return i

?

> Somehow that looks nicer with interfaces than with ABCs.

I think I've lost track of the difference.

Bill