[Python-Dev] metaclass insanity (original) (raw)

Guido van Rossum guido@python.org
Tue, 05 Nov 2002 10:38:41 -0500

> Can someone provide a reason why you'd want to use nested classes? > I've never felt this need myself. What are the motivations?

Yes. In Reality (a Twisted Matrix Labs project), a text-based virtual world simulation framework, we have a system that can automatically generate Interface definitions from classes (with a metaclass that builds them at class-def time).

(Beware of using a custom metaclass per class though -- it can prevent multiple inheritance, as I explained before in this thread.)

Twisted has an Interface/adapter/ componentized system similar to Zope's in twisted.python.components, and we use components/adapters to their fullest in Reality.

The problem is, our virtual worlds are persistent, so most of our objects get pickled (or serialized with some similar mechanism), and our objects hold references to these automatically-generated Interfaces. We would like to have these automatically-generated Interfaces as attributes of the classes they were generated for, but we have to use a hack that does something like:: setattr(subclass.module, interfaceName, NewInterface) This is really horrible, IMO.

Why didn't you choose to avoid nesting classes and use a naming convention instead, since the nesting causes problems?

Anyway, regardless that this may be a somewhat obscure use-case (although one we rely heavily on), It is wrong that str(klass) is lying about its location, and I seriously doubt fixing it would cause any problems.

Agreed. I'll respond to MvL's proposal.

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