  \textsc{Pykat} is a Python package which extends the popular optical
interferometer modelling software \textsc{Finesse}. It provides a more modern
and efficient user interface for conducting complex numerical simulations, as
well as enabling the use of Python's extensive scientific software ecosystem.
In this paper we highlight the relationship between \textsc{Pykat} and
\textsc{Finesse}, how it is used, and provide an illustrative example of how it
has helped to better understand the characteristics of the current generation
of gravitational wave interferometers.
