> Is there a difference if you do open(..., 'w')? It's a different enough operation that it may have a different error.

Oh, yes, I forgot the 'w' mode.

Mark, could you please run following test on Windows?

import os
open('foo', 'wb').close()
flags = os.O_RDWR | os.O_CREAT | os.O_EXCL | getattr(os, 'O_NOFOLLOW', 0) | getattr(os, 'O_BINARY', 0)
os.open('foo/bar', flags, 0o600)          # what raised?
os.open('nonexistent/bar', flags, 0o600)  # what raised?