python - Validate a argument value based on another argument -

i want validate argument based on value of argument.

samplescript.py      def parse_args():         parser = argparse.argumentparser()         parser.add_argument('-a', '--arg1', help="year-month or year")         parser.add_argument('-b', '--arg2', help="needs month argument arg1", action="store_true")         args = parser.parse_args()         return args       def main():         args = parse_args()       if __name__ == "__main__":         main() 

value arg1 can in format yyyy or yyyy-mm. however, when arg2 set, want make sure value arg1 passed in format 'yyyy-mm' , not 'yyyy'.

i.e when run python samplescript.py -a 2011, program should work fine. when run python samplescript.py -a 2011 -b, program should throw error since -a not in yyyy-mm format

is possible such validation using argparse ?