Return a relative version of a path

def __nt_relpath(path, start=os.curdir):
    """Return a relative version of a path"""

    if not path: raise ValueError("no path specified")

    start_list = os.path.abspath(start).split(os.sep)
    path_list = os.path.abspath(path).split(os.sep)
    if start_list[0].lower() != path_list[0].lower():
        unc_path, rest = os.path.splitunc(path)
        unc_start, rest = os.path.splitunc(start)
        if bool(unc_path) ^ bool(unc_start):
            raise ValueError("Cannot mix UNC and non-UNC paths (%s and %s)" \
                             % (path, start))
        else: raise ValueError("path is on drive %s, start on drive %s" \
                               % (path_list[0], start_list[0]))
    # Work out how much of the filepath is shared by start and path.
    for i in range(min(len(start_list), len(path_list))):
        if start_list[i].lower() != path_list[i].lower():
            break
        else: i += 1
        pass

    rel_list = [os.pardir] * (len(start_list)-i) + path_list[i:]
    if not rel_list: return os.curdir
    return os.path.join(*rel_list)