Generic metaclass to keep track of subclasses?

Question

I'm trying to write a generic metaclass to track subclasses

Since I want this to be generic, I didn't want to hardcode any class name within this metaclass, therefore I came up with a function that generates the proper metaclass, something like:

def make_subtracker(root):
    class SubclassTracker(type):
        def __init__(cls, name, bases, dct):
            print('registering %s' % (name,))
            root._registry.append(cls)
            super(SubclassTracker, cls).__init__(name, bases, dct)
    return SubclassTracker

This way I could invoke it to generate a metaclass for a specific root class with:

__metaclass__ = make_subtracker(Root)

Here is where I bump into a problem. I cannot do this:

class Root(object):
   _registry = []
   __metaclass__ = make_subtracker(Root)

...because Root is not defined yet when I use make_subtracker(Root). I tried adding the __metaclass__ attribute later, so that at least it can be applied in subclasses:

class Root(object):
   _registry = []

Root.__metaclass__ = make_subtracker(Root)

...but this doesn't work. __metaclass__ has a special processing when the class definition is read, as defined in Customizing class creation.

I'm looking for suggestions in order to do this (either change a class' metaclass at runtime in a way that it is applied to its subclasses, or any other alternative).

Answer 1

Python does this automatically for new-style classes, as mentioned in this answer to the similar queston How to find all the subclasses of a class given its name? here.