raising an exception when multiple inheritance involves same base

[Michael Hines]

Hello,
I have a class factory that supports single inheritance but it is
an error if the base appears twice. e.g
class Foo(hclass(h.Vector), hclass(h.List)):
should raise an exception since h.Vector and h.List are really the same
extension class, HocObject.

So far I have only been able to do this by iterating over __mro__ during
creation of an object as in:

def hclass(c):
    class hc(hoc.HocObject):
        def __new__(cls, *args, **kwds):
            m = False
            for x in cls.__mro__:
                if (type(x) == type(hc)):
                    if m == True:
                        raise HocError, 'Multiple inheritance...'
                    m = True
            kwds.update({'hocbase':cls.htype})
            return hoc.HocObject.__new__(cls, *args, **kwds)
    setattr(hc, 'htype', c)
    return hc

Is there a way to do the test earlier (e.g during creation of the Foo
class when hclass is called the second time
instead of during creation of a Foo object).
Just before the return hc, print hc.__mro__ yields for the second call:
(<class '__main__.hc'>, <type 'hoc.HocObject'>, <type 'object'>)
wheras in the __new__, print cls.__mro__ yields:
(<class '__main__.Foo'>, <class '__main__.hc'>, <class '__main__.hc'>,
<type 'hoc.HocObject'>, <type 'object'>)


Thanks,
Michael