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import threading
import pprint
class A:
 def __init__(self, **kw):
 pprint.pprint("a")
 super(A, self).__init__()
class B(threading.local, A):
 def __init__(self, **kw):
 pprint.pprint("b")
 super(B, self).__init__()
if __name__ == "__main__":
 B()
breaks (prints only b) in python 2. tested 2.7.2
works (prints b, a) in python 3, tested 3.2.2
threading.local is before A on purpose, to have different A attribute in different threads, not shown in the example.
caveat implementor: it may be impossible to support both super().__init__ and explicit threading.local.__init__ at the same time; explicit initialization is used far and wide in legacy code.

ah feeling stupid only minutes later.
A has to inherit from object() and then it works fine.