STARBURST99 is a population synthesis code tailored to predict the integrated properties or observational characteristics of star-forming galaxies. Here we present an update to STARBURST99 where we port the code to python, include new evolutionary tracks both rotating and non-rotating at a range of low metallicity environments. We complement these tracks with a corresponding grid of new synthetic SEDs. Additionally we include both evolutionary and spectral models of stars up to 300-500Msol. Synthesis models made with the python version of the code and new input stellar models are labelled pySTARBURST99. We make new predictions for many properties, such as ionising flux, SED, bolometric luminosity, wind power, hydrogen line equivalent widths and the UV beta-slope. These properties are all assessed over wider coverage in metallicity, mass and resolution than in previous versions of STARBURST99. A notable finding from these updates is an increase in H I ionising flux of 0.3 dex in the first 2Myr when increasing the upper mass limit from 120 to 300Msol. Changing metallicity has little impact on H I in the first 2Myr (range of 0.015 dex from Z = 0.02 to 0.0) but lower metallicities have higher H I by 1 dex (comparing Z = 0.02 to 0.0004) at later times, with Z = 0.0 having even higher H I at later times. Rotating models have significantly higher H I than their equivalent non-rotating models at any time after 2Myr. Similar trends are found for He I and He II, bolometric luminosity and wind momentum, with more complex relations found for hydrogen line equivalent widths and UV beta-slopes.
Comment: Accepted for publication in ApJS, 14 pages (50 including appendix), 15 figures (44 including appendix)