class A (object):
    def __init__(self, aFoo, aBar, aBaz):
        self._foo = aFoo
        self._bar = aBar
        self._baz = aBaz

class B (A):
    def __init__(self, aFoo=None, aBar=None, aBaz=None, aQuux=None, anA=None):
        if anA is not None:
            assert aFoo is aBar is aBaz is None
            super(B, self).__init__(anA._foo, anA._bar, anA._baz)
        else:
            super(B, self).__init__(aFoo, aBar, aBaz)
        self._quux = aQuux