(Give general background on UML.)
[http://sf.net/projects/pyut pyut] is a "Python UML Tool".
Pyreverse is supposed to be able to generate UML diagrams in XMI or ArgoUML format but the codebase has suffered bitrot as logilab is refactoring some common bits frm pylint.
PyNSource
pyIdea
Dia
sparxsystems?