[Python-Dev] type categories

[Martijn Faassen]

Guido van Rossum wrote:
> > Interfaces in Python are almost too trivial to understand, but
> > surprisingly useful. I imagine this is why so many smart Python
> > users don't get it; they either reject the notion because it seems
> > too trivial and 'therefore useless', or because they think it must
> > involve far more complication (static typing) and therefore it's too
> > complicated and not in the spirit of Python. :)
> 
> No, I think it's because they only work well if they are used
> pervasively (not necessarily everywhere).  That's why they work in
> Zope: not only does almost everything in Zope have an interface, but
> interfaces are used to implement many Zope features.

That is not true for Zope 2, and I do use interfaces in Zope 2. Zope 2
certainly doesn't use interfaces pervasively.

I use them pervasively in some of my code which is a framework
on top of Zope, which may weaken my argument, but it's still not
true interfaces are not useful unless they're used pervasively.
It's definitely a lot more powerful if you do, of course, though.

I also think it may help that one can declare a class implements an
interface outside said class itself, in a different section of the code.
I do not have any practical experience with that outside some Zope 3
hackery, however, so I can't really defend this one very well.

> I haven't made up my mind yet whether Python could benefit as much as
> Zope, but I am cautiosuly looking into adding something derived from
> Zope's interface package.  Jim & I have rather different ideas on what
> the ideal interfaces API should look like though, so it'll be a while.
> Maybe I should pull down the Twisted interfaces package and see how I
> like their subset (I'm sure it must be a subset -- the Zope package is
> a true kitchen sink :-).

It's an extremely small subset and very trivial, and last I checked
they used 'implements' in a different way than Zope, unfortunately (I pointed
it out and they may have fixed that by now, not sure).

But if you are looking for another API then the Twisted version doesn't
help (except for the inadvertent 'implements()' difference).

I don't consider the Zope 3 interface package to be a kitchen sink
myself, but I've been working with it for a while now. I would note
that some of its extensibility and introspection features is quite
useful when implementing Schema (a special kind of interfaces with
descriptions about non-method attributes). If a new package is to
be designed I hope that those use cases will be taken into account.

Regards,

Martijn