The test fails on Windows. In particular, this test:

self.assertEqual(posixpath.relpath(os.path.abspath("a")), "a")

fails. os.path.abspath("a") gives something like r"c:\python26\a". posixpath.relpath is then not able to cope with it. As a result, it returns r"c:\python26\a" as the relative path.

Using posixpath.abspath doesn't help, either, since that will give r"c:\python26/a" which relpath then still cannot process correctly.

One solution would be to use pass a POSIX path and start path to relpath; the other would be to use os.path in both cases.
os.path.relpath(os.path.abspath("a")) does indeed give "a" on Windows.