Re the patch: it looks like there's a debugging print left in it.  (Also, FYI, the patch did not apply cleanly using normal Unix patch; I had to strip the \r characters with (tr -d '\r' <patchcheck.diff); also it's best to not include Misc/NEWS changes in uploaded patches.) Otherwise, it should be good to go.

In general from Python's perspective, OS X is a POSIX-compliant (e.g. Unix-y) system, with "normal" Unix file system syntax and BSD-ish semantics.  So there's nothing special about os.path manipulations functions on OS X (platform="darwin") vs other Unix-based systems.  What can be confusing is that Classic Mac OS (Mac OS 9 and earlier) was not a Unix-based system and had a totally different file syntax.  Classic Mac OS has been obsolete for over a decade and nearly all traces of support for it (platform="mac") have been removed from Python 3 and Python 2.7, other than in os.path.