The test doesn’t really help me understand what this change does (i.e. what a human would see in an error case).

I’m also not sure if some of the changes in the various patches for bpo-9253 (especially in tests) could be useful here.

I'll check those out. I had a difficult time demonstrating (by assertion) the actual output (didn't find any examples) -- I'll bang against that and hopefully I'll make something more expressive :)

@merwok looks like they had the same idea I had (but a different implementation) -- I took inspiration from one of their tests for additional coverage and updated the tests to hopefully be more expressive (and have a better failure mode)

Code for this is already in _metavar_formatter:

        elif action.choices is not None:
            choice_strs = [str(choice) for choice in action.choices]
            result = '{%s}' % ','.join(choice_strs)

The metavar formatter can be called, if the parser is known (as self), using:

            default = self._get_default_metavar_for_positional(action)
            metavar, = self._metavar_formatter(action, default)(1)

Perhaps _get_action_name should be made into a method, or take the parser as an optional argument?

Add a space after comma. Why braces are used?

I think this is a notation for alternatives: it means one of foo or bar is required.

This is consistent with the rendering of choices:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--foo', choices=('baz', 'womp'))
parser.parse_args()

$ python test.py --help
usage: test.py [-h] [--foo {baz,womp}]

optional arguments:
  -h, --help        show this help message and exit
  --foo {baz,womp}

"foo, bar" is duplicated. And this error message looks confusing to me. What it means?

yeah I didn't really know what to do for this, it's essentially an anonymous positional choices argument. Personally I'd love a default dest='command' but I imagine that could cause some incompatibility?

Then this would read something like error: argument command: invalid choice ...