The documentation of os.path.join() uses os.sep because it's written from the perspective of the platform's os.path, which is posixpath on a POSIX system and ntpath on a Windows system.

If you need to work with POSIX paths in Windows, then use posixpath. It will of course use '/' as the path separator, since using backslash (os.sep) in a POSIX path is wrong. Vice versa, if you need to work with Windows paths in POSIX, use ntpath.