paul j3, thanks for reviewing my patch and giving me credit in your patch for another ticket.

Yeah, as you could see, the reason I return arg_parts and text is because the text still needs to undergo the cleanup process. You solved it by putting cleaning up in inner function.

I am thinking whether it is best to do "assert ' '.join(opt_parts) == opt_usage" inside _format_actions_usage helper function.

In that way, we can avoid returning the text. We can return only the arg_parts.

Anyway, my patch still got some unused variables, notably part_regexp and inner. My bad.

Let me check the code more deeply. See whether I can architect my patch in a better way. Maybe we can avoid building separate list inside _format_actions_usage.

Beside of that, this bug is not introduced solely by bracket character. It needs another non-space character on the right side of it.

This line is fine:
parser.add_argument ('--b', metavar="[innerpart] outerpart")

This line will fail the assertion:
parser.add_argument ('--b', metavar="[innerpart]outerpart")