Even after calling sdl2.mouse.SDL_ShowCursor(0), which effectively hides the cursor during runtime, when doing lengthy iterations (for my purposes, loading a large volume of images into RAM), the beachball cursor nonetheless appears.
It is critical that this doesn't happen because what I'm writing is an experiment for coginitive psychology on visual attention—this kind of visual distraction absolutely cannot occur.
Solved this myself by calling sdl2.SDL_PumpEvents() once per loop.
Pysdl2 on OSX How to prevent beachball cursor during long iteration loops