[Python-Dev] type categories

[Guido van Rossum]

[Guido]
> > I haven't given up the hope that inheritance and interfaces could use
> > the same mechanisms.  But Jim Fulton, based on years of experience in
> > Zope, claims they really should be different.  I wish I understood why
> > he thinks so.

[Ping]
> If i may hazard a guess, i'd imagine that Jim's answer would simply be
> that inheritance (of implementation) doesn't imply subtyping, and
> subtyping doesn't imply inheritance.

Well, yes, of course.  But I strongly believe that in *most* cases,
inheritance and subtyping go hand in hand.  I'd rather invent a
mechanism to deal with the exceptions rather than invent two parallel
mechanisms that must both be deployed separately to get the full
benefit out of them.

> That is, you may often want to re-use the implementation of one class
> in another class, but this doesn't mean the new class will meet all of
> the commitments of the old.  Conversely, you may often want to declare
> that different classes adhere to the same set of commitments (i.e.
> provide the same interface) even if they have different implementations.
> (A common situation where the latter occurs is when the implementations
> are written by different people.)

Nevertheless, these are exceptions to the general rule.

> > Agreeing on an ontology seems the hardest part to me.
> 
> Indeed.  One of the advantages of separating inheritance and subtyping
> is that this can give you a bit more flexibility in setting up the
> ontology, which may make it easier to settle on something good.

Really?  Given that there are no inheritance relationships between the
existing built-in types, I would think that you could define an
ontology consisting entirely of abstract types, and then graft the
concrete types on it.  I don't see what having separate interfaces
would buy you.  But perhaps you can give an example that shows your
point?

--Guido van Rossum (home page: http://www.python.org/~guido/)