Question

I want SuperClass12 to inherit from SuperClass1 and SuperClass2:

def getClass1(): 
    class MyMetaClass1(type):
        def __new__(cls, name, bases, dct):
            print dct.get("Attr","")+"Meta1"
            return super(MyMetaClass1, cls).__new__(cls, name, bases, dct)
    return MyMetaClass1

def getClass2(): 
    class MyMetaClass2(type):
        def __new__(cls, name, bases, dct):
            print dct.get("Attr","")+"Meta2"
            return super(MyMetaClass2, cls).__new__(cls, name, bases, dct)
    return MyMetaClass2    

class SuperClass1():
    __metaclass__ = getClass1()
    def fun1(self):
        pass

class SuperClass2():
    __metaclass__ = getClass2()
    def fun2(self):
        pass

class MyClass1(SuperClass1):
    Attr = "MC1"

class MyClass2(SuperClass2):
    Attr = "MC2"

def getClass12(): 
    class myMultiMeta(getClass1(),getClass2()):
        pass
    return myMultiMeta

class SuperClass12(SuperClass1,SuperClass2):
#class SuperClass12(): gives no errors in class construction but then 
#fun1() and fun2() are not members of SuperClass12. 
    __metaclass__ = getClass12()

class MyClass12(SuperClass12):
    Attr = "MC12"

Instance = MyClass12()
Instance.fun1()
Instance.fun2()

sadly I’ve got this error:

TypeError: Error when calling the metaclass bases metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

but I cannot understand why, since the metaclass myMultiMeta of my derived class SuperClass12 indeed is a subclass of bothe the metaclasses (MyMetaClass1,MyMetaClass2) of all its bases (SuperClass1,SUperClass2).

Answer 1

Remember that getClass1 and getClass2 return a new instance of the metaclass. Thus when you set __metaclass__ = getClass1(), that is a different metaclass than the one inherited by myMultiMeta.