def file_signature(file_name: str) -> Optional[Tuple]:
    """
    Return an identity signature for file name
    :param file_name: name of file
    :return: mode, size, last modified time if file exists, otherwise none
    """
    try:
        st = os.stat(file_name)
    except FileNotFoundError:
        return None
    return stat.S_IFMT(st.st_mode), st.st_size, st.st_mtime