You may want to use Doxygen, which generates nice diagrams. It's normally only for C++, but there are nice filters (for ex. http://i31www.ira.uka.de/~baas/pydoxy) that generates C++ header from python code that Doxygen can crunch.
Another solution is to use IDE such as Eric3 that can generate UML diags from source code on the fly. -- http://mail.python.org/mailman/listinfo/python-list
