http://xmlstar.sourceforge.net/ Not Python, but just the tool for the job.
For Python, 4Suite has some validators. But parts of it were written in C and might not be helpful if you want to look at the Python source code. You might want to check http://www.leuthe.homepage.t-online.de/minixsv/minixsv_overview.html for a pure Python sample. The page says it is not complete though. -- http://mail.python.org/mailman/listinfo/python-list
