In addition to NicDumZ's excellent modules, you may want to consider pyinotify for a rich (and richly documented) alternative. I think Nic's is simpler (at least for basical functionality), but I haven't studied it in detail -- I'm more familiar with pyinotify. BTW, interestingly, the latter is in pure Python (with ctypes
of course;-).
Since you did mention "should be able to work with unix", you should specify exactly which unix versions you need to support and study whether they offer similar functionality (and if so through what interface). For example, this man page for inotify says:
The inotify API is Linux specific. Some other systems provide
similar mechanisms, e.g., FreeBSD has kqueue, and Solaris has /dev/poll.
and Nic already mentioned FSEvents on MacOSX. Once you determine the C way to do it on a given system of interest, worst case, if there's no pre-made Python version, you can probably use ctypes
similarly to the way pyinotify does for inotify. BTW, for completeness: on Windows you might use ReadDirectoryChangesW
(typically you'd do that via either ctypes or the win32file file of the Win32 Extensions).