SAGE and Cython - Mathematics in Python

SAGE is an open source mathematics system with a Python-based interface (95% of the code is Python).

http://www.sagemath.org/download-source.html

You will see in the source code to SAGE that there are .pxd files, used to declare functions and classes in a C-extension module. This is a Cython convention. Cython makes it easy to write C-extensions for Python.