python - mutually exclusive arguments complains about action index

Question

I'm trying to group arguments such that the user can do either:

python sample.py scan -a 1 -b 2
or
python sample.pt save -d /tmp -n something

here is my code:

import argparse
if __name__ == '__main__':
    parser = argparse.ArgumentParser(
            description='this is the description'
            )
    parser.add_argument('op', choices=['scan','save'], help='operation', default='scan')
    root_group = parser.add_mutually_exclusive_group()

    group1 = root_group.add_argument_group('g1', 'scan')
    group1.add_argument('-a', help='dir1')
    group1.add_argument('-b', help='dir2')

    group2 = root_group.add_argument_group('g2', 'save')
    group2.add_argument('-d', help='dir')
    group2.add_argument('-n', help='name')

    args = parser.parse_args()
    print args

as I run python sample.py --help

I'm getting an error. Can someone please tell me how to fix it?

Traceback (most recent call last):
  File "sample.py", line 18, in <module>
    args = parser.parse_args()
  File "C:\Python27\lib\argparse.py", line 1688, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "C:\Python27\lib\argparse.py", line 1720, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "C:\Python27\lib\argparse.py", line 1926, in _parse_known_args
    start_index = consume_optional(start_index)
  File "C:\Python27\lib\argparse.py", line 1866, in consume_optional
    take_action(action, args, option_string)
  File "C:\Python27\lib\argparse.py", line 1794, in take_action
    action(self, namespace, argument_values, option_string)
  File "C:\Python27\lib\argparse.py", line 994, in __call__
    parser.print_help()
  File "C:\Python27\lib\argparse.py", line 2313, in print_help
    self._print_message(self.format_help(), file)
  File "C:\Python27\lib\argparse.py", line 2287, in format_help
    return formatter.format_help()
  File "C:\Python27\lib\argparse.py", line 279, in format_help
    help = self._root_section.format_help()
  File "C:\Python27\lib\argparse.py", line 209, in format_help
    func(*args)
  File "C:\Python27\lib\argparse.py", line 317, in _format_usage
    action_usage = format(optionals + positionals, groups)
  File "C:\Python27\lib\argparse.py", line 388, in _format_actions_usage
    start = actions.index(group._group_actions[0])
IndexError: list index out of range

and if I add action='store_const', the error goes away and a new error occurrs asking for 4 inputs.

Answer 1

Argparse does not seem to fully support adding a group into another group. This error happens because Argparse requires root_group to have some kind of action. A workaround would be adding an argument to the group:

import argparse

if __name__ == '__main__':
    parser = argparse.ArgumentParser(
            description='this is the description'
            )

    # This is now redundant. We can remove it
    # parser.add_argument('op', choices=['scan','save'], help='operation', default='scan')
    root_group = parser.add_mutually_exclusive_group()

    # Workaround
    root_group.add_argument('--scan', help='scan', action='store_true')
    root_group.add_argument('--save', help='save', action='store_true')

    group1 = root_group.add_argument_group('g1', 'scan')
    group2 = root_group.add_argument_group('g2', 'save')

    group1.add_argument('-a', help='dir1')
    group1.add_argument('-b', help='dir2')

    group2.add_argument('-d', help='dir', default='')
    group2.add_argument('-n', help='name')

    args = parser.parse_args()
    print args 

Notice that we are using --scan and --save. To avoid using the -- prefix, you may need the help of Sub-commands. Details can be found here.

Answer 2

Thanks to @skyline's link above, I got it working with subparsers:

import argparse
if __name__ == '__main__':
    parser = argparse.ArgumentParser(
            description='this is the description'
            )

    scan_parser = argparse.ArgumentParser(add_help=False)
    scan_parser.add_argument('-a', '--a', help='first num', required=True)
    scan_parser.add_argument('-b', '--b', help='second num', required=True)

    save_parser = argparse.ArgumentParser(add_help=False)
    save_parser.add_argument('-d', '--d', help='directory path', required=True)
    save_parser.add_argument('-n', '--n', help='name of the file', required=True)

    sp = parser.add_subparsers()

    sp_scan = sp.add_parser('scan', parents=[scan_parser], help='scans directories')
    sp_save = sp.add_parser('save', parents=[save_parser], help='saves something')
    args = parser.parse_args()
    print args