We present a numerical code to simulate maps of Galactic emission in intensity and polarisation at
microwave frequencies, aiding in the design of Cosmic Microwave Background experiments. This
Python code builds on existing efforts to simulate the sky by providing an easy-to-use interface and
is based on publicly available data from the \wmap\ and \planck\ satellite missions. We simulate
synchrotron, thermal dust, free-free, and anomalous microwave emission over the whole sky, in
addition to the Cosmic Microwave Background, and include a set of alternative prescriptions for the
frequency dependence of each component, for example polarised dust with multiple temperatures
and a decorrelation of the signals with frequency, which introduce complexity that is consistent with
current data. We also present a new prescription for adding small-scale realisations of these
components at resolutions greater than current all-sky measurements. The usefulness of the code
is demonstrated by forecasting the impact of varying foreground complexity on the recovered
tensor-to-scalar ratio for the LiteBIRD satellite. The code is available at
https://github.com/bthorne93/PySM_public.