Guido> For example, if we ever are going to add argument type declarations to Guido> Python, it will probably look like this: Guido> def foo(a: classA, b: classB): Guido> ...body... Guido> It would be convenient if this could be *defined* as Guido> assert isinstance(a, classA) and isinstance(b, classB) Guido> so that programs that have a simple class hierarchy can use their Guido> classes directly as argument types, without having to go through the Guido> trouble of declaring a parallel set of interfaces. Guido> I also think that it should be possible to come up with a set of Guido> standard "abstract" classes representing concepts like number, Guido> sequence, etc., in which the standard built-in types are nicely Guido> embedded. I agree completely. Any use of inheritance that satisfies Liskov substitutability will satisfy interface inheritance too, and although it is possible to think of uses of inheritance that arent substutitable, they're unusual enough that they should probably require (syntactic) special pleading, if only to alert the reader. -- Andrew Koenig, ark@research.att.com, http://www.research.att.com/info/ark