Bug report

Bug description:

I'm defining a argparse parser that works like vim:

support a file as a positional:

vim file.py

or reading from stdin:

cat file.py | vim -

So I write a mutually exclusive group:

import argparse
ap = argparse.ArgumentParser(description="fake vim")
mg = ap.add_mutually_exclusive_group(required=True)
mg.add_argument("input", nargs="?")
mg.add_argument("-", dest="from_stdin", action="store_true")
args = ap.parse_args()

When I use "-h" to show the help message, the "|" in usage line is dropped, as if the 2 arguments are not mutually exclusive:

usage: fake_vim.py [-h] [-] [input]

However, if I swap the 2 add_argument lines in the source code:

import argparse
ap = argparse.ArgumentParser(description="fake vim")
mg = ap.add_mutually_exclusive_group(required=True)
mg.add_argument("-", dest="from_stdin", action="store_true")
mg.add_argument("input", nargs="?")
args = ap.parse_args()

It magically behaves correctly:

usage: fake_vim.py [-h] (- | input)

CPython versions tested on:

3.9, 3.11

Operating systems tested on:

Windows

Linked PRs

• gh-113008: Correct argparse usage output for required, mutually exclusive groups #113085