A colleague is looking to generate UML class diagrams from heaps of Python source code. 
He's primarily interested in the inheritance relationships, and mildly interested in compositional relationships, and doesn't care much about class attributes that are just Python primitives.
The source code is pretty straightforward and not tremendously evil--it doesn't do any fancy metaclass magic, for example.  (It's mostly from the days of Python 1.5.2, with some sprinklings of "modern" 2.3ish stuff.)  
What's the best existing solution to recommend?
Certain classes of well-behaved programs may be diagrammable, but in the general case, it can't be done.  Python objects can be extended at run time, and objects of any type can be assigned to any instance variable.  Figuring out what classes an object can contain pointers to (composition) would require a full understanding of the runtime behavior of the program.
Python's metaclass capabilities mean that reasoning about the inheritance structure would also require a full understanding of the runtime behavior of the program.
To prove that these are impossible, you argue that if such a UML diagrammer existed, then you could take an arbitrary program, convert "halt" statements into statements that would impact the UML diagram, and use the UML diagrammer to solve the halting problem, which as we know is impossible.
What's the best way to generate a UML diagram from Python source code?