[Tutor] inheritance/classmethods/metaclasses

[Eric Abrahamsen]

I'm probably in over my head here, but I really want to know how this  
works, and I hope someone will be willing to take a moment to explain  
it...

I'm making a few classes for a very simple ORM, where the classes  
reflect objects that I'm persisting via pickle. I'd like to add and  
delete instances via add() and delete() classmethods, so that the  
classes can keep track of the comings and goings of their instances.

My main question is, how can I abstract this classmethod behavior for  
several classes, either into a metaclass, or a super class? I've done  
some very scary reading, and some fruitless experimentation, and am  
over my head. I know the answer is probably "do something else  
entirely", but I'd really like to know how this is supposed to work.  
What I'd like is a Model class, which provides an add() classmethod  
like this:

class Model(object):
@classmethod
def add(cls, *args)
     inst = cls(args)
     cls.instances.append(inst)

Then, say, a File class:

class File():
     # inherit from Model, or use Model as a metaclass?
     instances = []
     def __init__(self, *args)
         self.filename = args[0]
         self.keyword = args[1]

The desired behavior, of course, is this:

File.add('somefilename','keywordforthisfile')

and to be able to extend this behavior to other classes.

Any pointers would be much appreciated...

Eric