class abase:
    def from_base(self): return True

class meta1(type):
    def __init__(cls, name, bases, dict):
        super(meta1, cls).__init__(name, bases, dict)
        setattr(cls, '__bases__', getattr(cls, '__bases__', tuple()) + (abase,))

class meta2(type):
    def __init__(cls, name, bases, dict):
        bases += (abase,)
        super(meta2, cls).__init__(name, bases, dict)

__metaclass__ = meta1

class A:
    def __init__(self):
        print self.from_base()

__metaclass__ = meta2

class B:
    def __init__(self):
        print self.from_base()

A()
B()