Windows doesn't use the fs encoding at all until Python code requests/provides something in bytes. Except for the caching in fsencode/fsdecode, there's no problem setting it once at the start of your program (and it can only be set once - there's no parameter and it cannot be undone).

What I'm most interested in is whether caching the encoding in fsencode/fsdecode is actually an optimization - if not, remove it, and if so make a way to reset it. I'll get around to this sooner or later but I don't want to stop someone else from doing it.