[Zope3-dev] Re: Interface support in Python? (was Re: [Zope3-dev] Proposal: Improving on __implements__)

[Jim Fulton]

Phillip J. Eby wrote:

...

> I think for me, another issue would most definitely be proper support 
> for metaclasses, i.e. spelling the difference between what classes and 
> instances implement.  While it may be true that "most people" will not 
> implement metaclasses, it is still quite possible for them to be bitten 
> by the difference when using metaclasses provided by someone else.  
> (Currently, __class_implements__ works reasonably well, but it can get 
> really hairy when you have metametaclasses in play.)

I agree. I'll note that you don't need meta classes to have classes
that implement interesting interfaces. Classes do have some methods
of their own, most notably __call__, and can habve more with class
and static methods.

> If we were writing the interface package today -- assuming Python 2.2 
> and up -- I'd suggest always asking an object's class for what it 
> implements, passing in either the instance or the class.  That seems to 
> be the only way to be completely unambiguous as to whose interfaces 
> you're asking for, when the object in question might itself be a class 
> or type.  An __implements__ descriptor can only be unambiguous if it's 
> defined on the meta-most metaclass of the system.  So to do it cleanly 
> it'd have to be part of type 'type', which implies a language change.

I'm not sure what you just said. ;)

I'll note that it is possible to design a descriptor for __implements__
that will give different answers depending on whether you access it
through a class or an instance.  You don't need __class_implements__.
And you can get an object's implementation specification unambiguously.
No meta-class is necessary.  To make this work, you don't want people
setting __implements__ directly.

> Offhand, this sounds like something with no chance of getting in 2.3, 
> and therefore unable to significantly affect Zope 3, but maybe I'm 
> missing something here.

It's hard to say. It won't get into Python 2.3 if it involves syntax
changes. Even if it doesn't get into Python 2.3, I think we'd be in a
much better position to reason about near-term changes if we knew what
it would look like in some future version.

Jim

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