Thanks Ezio.

I've updated the patch to incorporate your suggestions.

Note that I too have only tested it on Linux, but I tested both
posixpath and ntpath (and there is no OS-specific code, except for the
filenames themselves).

I'm not sure if using assertTrue(isinstance ...) is better than
assertEqual(type ...), because the type equality checking produces this
error:
AssertionError: <type 'str'> != <type 'unicode'>
while isinstance produces this unhelpful error:
AssertionError: False is not True

But oh well, I made the change anyway as most test cases use isinstance.