metaclass.py

class Manager(object):
    classes = set()


class Meta(type):
    def __new__(cls, name, bases, d):
        print("name:", name, "\nbases:", bases, "\ndict: ", d)
        if name is not "Base":
            Manager.classes.add(d['data'])
        return type.__new__(cls, name, bases, d)


class Base(object, metaclass=Meta):
    data = None


class ClassA(Base):
    data = 'a'


class ClassB(Base):
    data = 'b'


if __name__ == "__main__":
    print(Manager.classes)