gistfile1.py

class MetaA(type):
    def __call__(cls, *args, **kwargs):
        for name, class in cls.__dict__.iteritems():
            if issubclass(class.__class__, B):
                # Do something

        return # class A with modified fields

class B(object):
    ....

class A(object):
    __metaclass__ = MetaA

    abra = B(....)
    kadabra = B(....)