argparse drops "|" when a positional, then a special "-" argument (order-sensitive), are defined in a mutually exclusive group

[keeeeymann]

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

[savannahostrowski]

I can repro with 3.9, 3.10 and 3.11.

[hpaulj]

two things

A mutually exclusive group can handle only one positional argument, and it should be the last one added to the group.. I don't recall if that's more a limitation in the testing or the usage. Usage formatting is know to be brittle, especially with regard to the MXGs.

A bare '-' dest is not encouraged. It may be possible, but usually should something like '-f', dash plus single character.

Usage formatting needs a complete rewrite - one of these days.

[serhiy-storchaka]

Fixed in 3.14. Not sure if it's worth doing the backport -- it's only a minor improvement to the help output, and there's already a known workaround.

As for the original example, I do not recommend to use option "-" to solve this problem. It would work weird if you have more than one positional arguments. It is easier to simply check the positional argument value after parsing:

args.from_stdin = (args.input == '-')