pygeodyn is a sequential geomagnetic data assimilation package written in Python. It gives access to the core surface dynamics, controlled by geomagnetic observations, by means of a stochastic model anchored to geodynamo simulation statistics. pygeodyn aims at giving access to a user-friendly and flexible data assimilation algorithm. It is designed to be tunable by the community by different means: possibility to use embedded data and priors, or to supply custom ones; tunable parameters through configuration files; adapted documentation for several user profiles. In addition, output files are directly 5 supported by the package webgeodyn that provides a set of visualisation tools to explore the results of computations.