Triple inheritance causes metaclass conflict… Sometimes

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The error message indicates that you have two conflicting metaclasses somewhere in your hierarchy. You need to examine each of your classes and the QT classes to figure out where the conflict is.

Here’s some simple example code that sets up the same situation:

class MetaA(type):
    pass
class MetaB(type):
    pass
class A:
    __metaclass__ = MetaA
class B:
    __metaclass__ = MetaB

We can’t subclass both of those classes directly, because python wouldn’t know which metaclass to use:

>>> class Broken(A, B): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
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

What the error is trying to tell us is that we need to resolve the conflict between the two metaclasses by introducing a third metaclass that is a subclass of all the metaclasses from the base classes.

I’m not sure that’s any clearer than the error message itself, but basically, you fix it by doing this:

class MetaAB(MetaA, MetaB):
    pass

class Fixed(A, B):
    __metaclass__ = MetaAB

This code now compiles and runs correctly. Of course, in the real situation, your conflict-resolving metaclass would have to decide which of the parent metaclass behaviors to adopt, which you’ll have to figure out for yourself from your application’s requirements.

Bear in mind that your inherited class only gets one of the two metaclass.__init__ methods, which sometimes do all the work, so in a lot of cases, you are going to have to add an __init__ that calls into both in some way that helps them get along.

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