Modern programs are increasingly multilanguage, to benefit from each programming language’s advantages and to reuse libraries. For example, developers may want to combine high-level Python code with low-level, performance-oriented C code. In fact one in five of the 200 most-downloaded Python libraries available on GitHub contain C code. Static analyzers tend to focus on a single language, and may use stubs to model the behavior of foreign function calls. However, stubs are costly to implement and undermine the soundness of analyzers. In this work we design a static analyzer by abstract interpretation that can handle Python programs calling C extensions. 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. This approach allows sharing between abstract domains of different languages. Our analyzer can tackle tests of real-world libraries a few thousand lines of C and Python long.