isabs also fails for device paths such as r"\\.\C:", which is an absolute path for opening the C: volume. UNC and device paths (i.e. \\server, \\?, and \\.) should always be considered absolute. Only logical drives (i.e. C:, D:, etc) support drive-relative paths.

Also, join() needs to be smarter in this case:

    >>> os.path.join(r'\\.\C:', 'spam')
    '\\\\.\\C:spam'

It doesn't insert a backslash because the 'drive' ends in a colon. It needs to always insert a backslash for UNC paths, which cannot be reliably identified by checking whether the last character of the drive is a colon.