SHTools & pyshtools: Tools for Working with Spherical Harmonics
SHTOOLS/pyshtools is a Fortran-95/Python library that can be used to perform spherical harmonic transforms, multitaper spectral analyses, expansions of functions into Slepian bases, and standard operations on global gravitational and magnetic field data.
pyshtools's documentation: https://shtools.github.io/SHTOOLS/
SHTOOLS documentation: https://shtools.github.io/SHTOOLS/index-fortran.html
Wieczorek, M. A., & Meschede, M. (2018). Shtools: Tools for working with spherical harmonics. Geochemistry, Geophysics, Geosystems, 19(8), 2574-2592. https://doi.org/10.1029/2018GC007529