Generic metaclass to keep track of subclasses?

Question

I'm trying to writing a generic metaclass for tracking 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 http://docs.python.org/reference/datamodel.html#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

Here's something I've been playing around with (that works):

def sublass_registry():
    ''' Create a metaclass to register subclasses '''

    class SublassRegistryMeta(type):
        def __init__(cls, name, bases, classdict):
            if classdict.get('__metaclass__') is SublassRegistryMeta:
                SublassRegistryMeta.lineage = [cls] # put root class at head of a list
            else:
                # sublclasses won't have __metaclass__ explicitly set to this class
                # we know they're subclassees because this ctor is being called for them
                SublassRegistryMeta.lineage.append(cls) # add subclass to list
            type.__init__(cls, name, bases, classdict)

    return SublassRegistryMeta

def subclasses(cls):
    ''' Return a list containing base and subclasses '''

    try:
        if cls.__metaclass__.lineage[0] is cls: # only valid for a root class
            return cls.__metaclass__.lineage
    except AttributeError:
        pass
    return None

class Car(object): # root class
    __metaclass__ = sublass_registry()

class Audi(Car): # inherits __metaclass__
    pass

class Ford(Car): # inherits __metaclass__
    pass

class Audi2(Audi): # sub-subclass also inherits __metaclass__
    pass

print subclasses(Car)
# [<class '__main__.Car'>, <class '__main__.Audi'>, <class '__main__.Ford'>, <class '__main__.Audi2'>]
print subclasses(Audi)
# None

Answer 2

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

Answer 3

I think you want something like this (untested):

class SubclassTracker(type):
    def __init__(cls, name, bases, dct):
        if not hasattr(cls, '_registry'):
            cls._registry = []
        print('registering %s' % (name,))
        cls._registry.append(cls)
        super(SubclassTracker, cls).__init__(name, bases, dct)

Then, for Python 2, you can invoke it like:

class Root(object):
    __metaclass__ = SubclassTracker

for Python 3

class Root(object, metaclass=SubclassTracker):

Note that you don't need to stick the _registry attribute on there because stuff like that is what metaclasses are for. Since you already happen to have one laying around... ;)

Note also that you might want to move the registration code into an else clause so that the class doesn't register itself as a subclass.