Python argparse mutual exclusive group

What I need is:

    pro [-a xxx | [-b yyy -c zzz]]

I tried this but does not work. Could someone help me out?

    group= parser.add_argument_group('Model 2')
    group_ex = group.add_mutually_exclusive_group()
    group_ex.add_argument("-a", type=str, action = "store", default = "", help="test")
    group_ex_2 = group_ex.add_argument_group("option 2")
    group_ex_2.add_argument("-b", type=str, action = "store", default = "", help="test")
    group_ex_2.add_argument("-c", type=str, action = "store", default = "", help="test")


add_mutually_exclusive_group doesn't make an entire group mutually exclusive. It makes options within the group mutually exclusive.

What you're looking for is subcommands. Instead of prog [ -a xxxx | [-b yyy -c zzz]], you'd have:

      command 1 
        -a: ...
      command 2
        -b: ...
        -c: ...

To invoke with the first set of arguments:

    prog command_1 -a xxxx

To invoke with the second set of arguments:

    prog command_2 -b yyyy -c zzzz

You can also set the sub command arguments as positional.

    prog command_1 xxxx

Kind of like git or svn:

    git commit -am
    git merge develop

Working Example

    # create the top-level parser
    parser = argparse.ArgumentParser(prog='PROG')
    parser.add_argument('--foo', action='store_true', help='help for foo arg.')
    subparsers = parser.add_subparsers(help='help for subcommand')

    # create the parser for the "command_1" command
    parser_a = subparsers.add_parser('command_1', help='command_1 help')
    parser_a.add_argument('a', type=str, help='help for bar, positional')

    # create the parser for the "command_2" command
    parser_b = subparsers.add_parser('command_2', help='help for command_2')
    parser_b.add_argument('-b', type=str, help='help for b')
    parser_b.add_argument('-c', type=str, action='store', default='', help='test')

Test it

    >>> parser.print_help()
    usage: PROG [-h] [--foo] {command_1,command_2} ...

    positional arguments:
                            help for subcommand
        command_1           command_1 help
        command_2           help for command_2

    optional arguments:
      -h, --help            show this help message and exit
      --foo                 help for foo arg.

    >>> parser.parse_args(['command_1', 'working'])
    Namespace(a='working', foo=False)
    >>> parser.parse_args(['command_1', 'wellness', '-b x'])
    usage: PROG [-h] [--foo] {command_1,command_2} ...
    PROG: error: unrecognized arguments: -b x

Good luck.