Hi, I am using argparse to handle cmd args, I wanna if there is no args specified, then print the help message, but now the parse will output a error, and then exit. my code is:

def main():
    print "in abing/start/main"
    parser = argparse.ArgumentParser(prog="abing")#, usage="%(prog)s <command> [args] [--help]")
    parser.add_argument("-v", "--verbose", action="store_true", default=False, help="show verbose output")

    subparsers = parser.add_subparsers(title="commands")

    bkr_subparser = subparsers.add_parser("beaker", help="beaker inspection")
    bkr_subparser.set_defaults(command=beaker_command)
    bkr_subparser.add_argument("-m", "--max", action="store", default=3, type=int, help="max resubmit count")
    bkr_subparser.add_argument("-g", "--grain", action="store", default="J", choices=["J", "RS", "R", "T", "job", "recipeset", "recipe", "task"], type=str, help="resubmit selection granularity")
    bkr_subparser.add_argument("job_ids", nargs=1, action="store", help="list of job id to be monitored")

    et_subparser = subparsers.add_parser("errata", help="errata inspection")
    et_subparser.set_defaults(command=errata_command)
    et_subparser.add_argument("-w", "--workflows", action="store_true", help="generate workflows for the erratum")
    et_subparser.add_argument("-r", "--run", action="store_true", help="generate workflows, and run for the erratum")
    et_subparser.add_argument("-s", "--start-monitor", action="store_true", help="start monitor the errata system")
    et_subparser.add_argument("-d", "--daemon", action="store_true", help="run monitor into daemon mode")
    et_subparser.add_argument("erratum", action="store", nargs=1, metavar="ERRATUM", help="erratum id")

    if len(sys.argv) == 1:
        parser.print_help()
        return

    args = parser.parse_args()
    args.command(args)

    return

how can I do that? thanks.