Python Metaclass woes.

metaclass_woes.py

storage = {}

class RegistryMeta(type):
    def __init__(cls, name, bases, attrs):
        super(RegistryMeta, cls).__init__(name, bases, attrs)

        id = getattr(cls, 'id', None)
        if not id:
            return

        if id in storage:
            raise KeyError("Already registered: %s" % name)

        storage[id] = cls


class Base(object):
    __metaclass__ = RegistryMeta  # <-- Works without this.

    def __init__(self):
        self.foo = self._compute_foo()

    def _compute_foo(self):
        return 42


class Mixin(object):
    def _compute_foo(self):
        return 69


class C(Base, Mixin):
    pass


assert C().foo == 69