def is_newer(source, target):
    """Return true if 'source' exists and is more recently modified than
    'target', or if 'source' exists and 'target' doesn't.  Return false if
    both exist and 'target' is the same age or younger than 'source'.
    Raise ValueError if 'source' does not exist.
    """
    if not os.path.exists(source):
        raise ValueError("file '%s' does not exist" % source)
    if not os.path.exists(target):
        return 1

    from stat import ST_MTIME
    mtime1 = os.stat(source)[ST_MTIME]
    mtime2 = os.stat(target)[ST_MTIME]

    return mtime1 > mtime2
Return true if 'source' exists and is more recently modified than
    'target', or if 'source' exists and 'target' doesn't.  Return false if
    both exist and 'target' is the same age or younger than 'source'.
    Raise ValueError if 'source' does not exist.