Porting library from Java to Python
Jython's not what you're looking for in the final solution, but it will make the porting go much smoother.
My approach would be:

If there are existing tests (unit or otherwise), rewrite them in Jython (using Python's unittest)
Write some characterization tests in Jython (tests that record the current behavior)
Start porting class by class:


For each class, subclass it in Jython and port the methods one by one, making the method in the superclass abstract
After each change, run the tests!

You'll now have working Jython code that hopefully has minimal dependencies on Java.
Run the tests in CPython and fix whatever's left.
Refactor - you'll want to Pythonify the code, probably simplifying it a lot with Python idioms. This is safe and easy because of the tests.

I've this in the past with great success.
I'm about to port a smallish library from Java to Python and wanted some advice (smallish ~ a few thousand lines of code).  I've studied the Java code a little, and noticed some design patterns that are common in both languages.  However, there were definitely some Java-only idioms (singletons, etc) present that are generally not-well-received in Python-world.
I know at least one tool (j2py) exists that will turn a .java file into a .py file by walking the AST.  Some initial experimentation yielded less than favorable results.
Should I even be considering using an automated tool to generate some code, or are the languages different enough that any tool would create enough re-work to have justified writing from scratch?  
If tools aren't the devil, are there any besides j2py that can at least handle same-project import management?  I don't expect any tool to match 3rd party libraries from one language to a substitute in another.