SymPy is a Python library for symbolic mathematics. It aims to become a full-featured computer algebra system (CAS) while keeping the code as simple as possible in order to be comprehensible and easily extensible.
Ondřej started the project in 2005, and since then developed a large user and developer community. He handed over the leadership to Aaron Meurer in 2011.
SymPy has participated in every Google Summer of Code since 2007 and Ondřej has been either a mentor or an organization admin (together with Aaron Meurer). You can see here for full details.
Project link: https://sympy.org