[issue17218] support title and description in argparse add_mutually_exclusive_group

[paul j3]

paul j3 added the comment:

Using a mutually_exclusive_group is a little more complicated than I implied in the previous post.

    p=argparse.ArgumentParser()
    g=p.add_mutually_exclusive_group()
    # currently the code objects to 'title' and 'description' keywords
    g.add_argument('--foo')
    g.add_argument('--bar')
    # but a title can be added after creation
    g.title='test'
    g.description='description'
    # now add the group to the list that is used for help formatting
    p._action_groups.append(g)
    p.print_help()

producing:

    usage: ipython [-h] [--foo FOO | --bar BAR]
    optional arguments:
      -h, --help  show this help message and exit
      --foo FOO
      --bar BAR
    test:
      description
      --foo FOO
      --bar BAR

Now the arguments appear in both the 'optional arguments' group (a default one), and the new group.  That's not what we want.

So the mutually_exclusive_group has to be changed so it accepts title and description.  And it also has to block the addition of arguments to one of the existing groups.

A key difference is in how _add_action is implemented for the 2 group classes:

For argument_group:

    def _add_action(self, action):
        action = super(_ArgumentGroup, self)._add_action(action)
        self._group_actions.append(action)
        return action

for mutually exclusive group

    def _add_action(self, action):
        ...
        action = self._container._add_action(action)
        self._group_actions.append(action)
        return action

The first uses 'super' to add the action to itself.  The second adds the action to its 'container'.

That difference allows you to add a mutually_exclusive_group to an argument_group (or to another mutually_exclusive_group), but you can't add an argument_group to another argument_group (no nesting).

I don't like the idea of using a different _add_action method depending on whether group has a 'title' or not.  That's too kludgy.

Another possibility is to have 'parser.add_mutually_exclusive_group()' do what I first demonstrated - first create an 'argument_group' with the title, and add the mutually_exclusive_group to that.  This is still kludgy, but the change is limited to one function.  For demonstration purposes it probably could be implemented in a new function (add_titled_mutually_exclusive_group).

----------

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17218>
_______________________________________