Mopsa is a conservative static analysis platform, independent of language and abstraction choices. Developers are free to add arbitrary abstractions (numeric, pointer, memory, etc.) and syntax iterators for new languages. Mopsa encourages the development of independent abstractions which can cooperate or be combined to improve precision. In this talk, we will show how Mopsa analyses Python programs calling C libraries. It analyses directly and fully automatically both the Python and the C source codes. It reports runtime errors that may happen in Python, in C, and at the interface. We implemented our analysis in a modular fashion: it reuses off-the-shelf C and Python analyses written in the same analyzer. Our analyzer can tackle tests of real-world libraries a few thousand lines of C and Python long.