def _add_message_file(arg_parser, help_text):
    """
    Add mesage file argument with help_text to arg_parser.
    :param arg_parser: ArgumentParser parser to add this argument to.
    :param help_text: str: help text for this argument
    """
    arg_parser.add_argument('--msg-file',
                            type=argparse.FileType('r'),
                            help=help_text)