Python Optional Argument Pair

Question

I'm using the argparse module to get two optional command line arguments:

parser.add_argument('start_date', nargs='?', metavar='START DATE',
                   help='start date in YYYY-MM-DD')
parser.add_argument('end_date', nargs='?', metavar='END DATE',
                   help='end date in YYYY-MM-DD')

which gives

> python test_arg.py -h
usage: test_arg.py [-h] [START DATE] [END DATE]

However I want the pair of optional arguments (START DATE and END DATE), if provided at all, to be provided together. Something like along this line:

usage: test_arg.py [-h] [START_DATE END_DATE]

Is it possible with argparse?

Answer 1

The closest I can come up with is:

parser=argparse.ArgumentParser()
parser.add_argument('--dates', nargs=2, metavar=('START DATE','END_DATE'),
                   help='start date and end date in YYYY-MM-DD')
print(parser.format_help())

which produces

usage: stock19805170.py [-h] [--dates START DATE END_DATE]

optional arguments:
  -h, --help            show this help message and exit
  --dates START DATE END_DATE
                        start date and end date in YYYY-MM-DD

There isn't a way of specifying - 'require these 2 arguments together'. nargs=2 specifies 2 arguments, but doesn't make them optional (a nargs=[0,2] form has been proposed but not incorporated into any distribution). So --dates is needed to make it optional. To produce this help, the metavar must be a tuple (try it with a list to see what I mean). And that tuple metavar only works for optionals (not positionals).

Answer 2

I think the only way to do this is to do the check yourself:

if (not parser.start_date) != (not parser.end_date):
    print("Error: --start_date and --end_date must be used together.")
    arg_parser.print_usage()
    sys.exit(-1)

Unfortunately, that's not reflected in the help message.