small potential issue here is that os.path.normpath is plaform independent (on windows,
it will replace '/' with '\'). 

An unlikely issue for us, of course, but posixpath.normpath() is a more predictable
version

(and all of the urlparse code expects a full url, with schema://etc, so is kind of
useless for a subpath,so posixpath.normpath() is probably easiest thing)