There is QuantLib at http://quantlib.org/ . The site says "QuantLib is written in C++ with a clean object model, and is then exported to different languages such as Python, Ruby, and Scheme." I have not tried it -- if it is easily usable from Python please write back to c.l.p.
There is a Python Finance email list at http://www.reportlab.co.uk/mailman/listinfo/python-finance . In calculations involving bonds one needs arrays to store payment dates and amounts etc., for which numarray should be used instead of Python lists, for efficiency. -- http://mail.python.org/mailman/listinfo/python-list
