This looks like a documentation bug. Functions in module os are usually just thin wrappers around the underlying OS functions, and POSIX states that doing nothing is the correct thing to do here. (It is arguably a bug in early Unix implementations that got mistakenly codified as part of POSIX, and it is certainly inconsistent with the POSIX requirement that the mv command *must* remove the source file in this case, but there is nothing Python can do about that.)