Using mutually exclusive between groups

Question

I'm trying to have a mutually exclusive group between different groups: I have the arguments -a,-b,-c, and I want to have a conflict with -a and -b together, or -a and -c together. The help should show something like [-a | ([-b] [-c])].

The following code does not seem to do have mutually exclusive options:

import argparse
parser = argparse.ArgumentParser(description='My desc')
main_group = parser.add_mutually_exclusive_group()
mysub_group = main_group.add_argument_group()
main_group.add_argument("-a", dest='a', action='store_true', default=False, help='a help')
mysub_group.add_argument("-b", dest='b', action='store_true',default=False,help='b help')
mysub_group.add_argument("-c", dest='c', action='store_true',default=False,help='c help')
parser.parse_args()

Parsing different combinations - all pass:

> python myscript.py -h
usage: myscript.py [-h] [-a] [-b] [-c]

My desc

optional arguments:
  -h, --help  show this help message and exit
  -a          a help
> python myscript.py -a -c
> python myscript.py -a -b
> python myscript.py -b -c

I tried changing the mysub_group to be add_mutually_exclusive_group turns everything into mutually exclusive:

> python myscript.py -h
usage: myscript.py [-h] [-a | -b | -c]

My desc

optional arguments:
  -h, --help  show this help message and exit
  -a          a help
  -b          b help
  -c          c help

How can I add arguments for [-a | ([-b] [-c])]?

Answer 1

So, this has been asked a number of times. The simple answer is "with argparse, you can't". However, that's the simple answer. You could make use of subparsers, so:

import argparse
parser = argparse.ArgumentParser(description='My desc')
sub_parsers = parser.add_subparsers()
parser_a = sub_parsers.add_parser("a", help='a help')
parser_b = sub_parsers.add_parser("b", help='b help')
parser_b.add_argument("-c", dest='c', action='store_true',default=False,help='c help')
parser.parse_args()

You then get:

$ python parser -h
usage: parser [-h] {a,b} ...

My desc

positional arguments:
  {a,b}
    a         a help
    b         b help

optional arguments:
  -h, --help  show this help message and exit

and:

$ python parser b -h
usage: parser b [-h] [-c]

optional arguments:
  -h, --help  show this help message and exit
  -c          c help

If you would prefer your arguments as stated in the question, have a look at docopt, it looks lovely, and should do exactly what you want.

Answer 2

argument_groups don't affect the parsing. They just contribute to the help formatting. So defining a group within an mutually_exclusive_group does not help with this problem.

There is a proposed patch, http://bugs.python.org/issue10984, 'argparse add_mutually_exclusive_group should accept existing arguments to register conflicts', that would allow you to define two mutually_exclusive_groups, one [-a | -b] and the other [-a | -c]. Creating a 2nd group that includes an argument (-a) that is already defined is the trivial part of this patch. Producing a meaningful usage line is more difficult, and required a rewrite of several HelpFormatter methods.

import argparse
parser = argparse.ArgumentParser(description='My desc')
group1 = parser.add_mutually_exclusive_group()
action_a = group1.add_argument("-a", dest='a', action='store_true', default=False, help='a help')
group1.add_argument("-b", dest='b', action='store_true',default=False,help='b help')

group2 = parser.add_mutually_exclusive_group()
group2.add_argument("-c", dest='c', action='store_true',default=False,help='c help')
group2._group_actions.append(action_a) # THE KLUDGE

print parser.format_usage() 
# usage: stack16769409.py [-h] [-a | -b] [-c] 

args  = parser.parse_args()

Usage does not show the 2 groups correctly. But it does accept -b -c, while objecting to -a -b and -a -c. But you can write a custom usage line.