gh-157127: Reword the argparse mutually exclusive group error message

[lazerg]

When a required mutually exclusive group is not satisfied, argparse joins the
argument names with a plain space:

error: one of the arguments --stdout repository is required

With a positional in the group this reads as broken grammar, since nothing
separates one name from the next.

The error for missing required arguments a few lines above in
_parse_known_args() puts its names at the end of the sentence and joins them
with ', '. This makes the mutually exclusive group error do the same:

error: one of the following arguments is required: --stdout, repository

The doc example and the three affected test assertions are updated to match.

Fixes #157127

• Issue: argparse exclusive argument group error unintelligible #157127

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #34466763 | 📁 Comparing 3f7606e against main (23180c5)

  🔍 Preview build  

10 files changed · ± 10 modified

± Modified

• c-api/module.html
• c-api/type.html
• library/argparse.html
• library/math.html
• library/os.html
• library/socket.html
• using/configure.html
• using/ios.html
• whatsnew/3.16.html
• whatsnew/changelog.html

[picnixz]

I suggest to reweite the error message as follows: "one of the following arguments is required: arg1, arg2, arg3".

Otherwise could we have some examples of what other CLIs in other packages format such errors (if any?) it would be good to take inspiration from them when possible

[lazerg]

@picnixz Reworded in 3f7606e:

PROG: error: one of the following arguments is required: --foo, --bar

It now matches the error argparse raises a few lines above for plain required arguments, the following arguments are required: --foo, --bar, so the two read the same way.

For the other question, here is how three CLIs I checked format it:

• Cobra (docker, kubectl, gh): at least one of the flags in the group [--foo --bar] is required
• clap (Rust): the following required arguments were not provided:, then <--foo|--bar> on the next line
• argparse itself, non-exclusive case: the following arguments are required: --foo, --bar

All three put the names at the end and keep a visible separator between them. Brackets, pipes or commas, but never a bare space. The new message follows that.