When you did: b = a[:] b was then a copy of a, rather than just a reference to the same a. But what does a contain? It contains two sublists -- that is, it contains references to two sublists. So b, which is now a copy of a, contains copies of the two references to the same two sublists.
What you need to do instead is: b = copy.deepcopy(a) to get you what you actually want. -- http://mail.python.org/mailman/listinfo/python-list
