gh-132558: allow choices to be specified as strings in presence of a type argument

[hansthen]

As a motivating example, consider the following program:

import argparse

parser = argparse.ArgumentParser()

day_names = ["mo", "tu", "we", "th", "fr", "sa", "su"]
days = [1, 2, 3, 4, 5, 6, 7]

def name_day(value):
    return day_names.index(value) + 1

parser.add_argument(
    '--day',
    type=name_day,
    choices=days,
)
parser.parse_args()

This gives the following usage string:

usage: example.py [-h] [--day {1,2,3,4,5,6,7}]

With the proposed pull request, the program can be rewritten as follows:

import argparse

parser = argparse.ArgumentParser()

day_names = ["mo", "tu", "we", "th", "fr", "sa", "su"]

def name_day(value):
    return day_names.index(value) + 1

parser.add_argument(
    '--day',
    type=name_day,
    choices=days,
)
parser.parse_args()

With the following usage string:

usage: example_new.py [-h] [--days {mo,tu,we,th,fr,sa,su}]

I took care to ensure backward compatibility including by adding a feature flag for the behavior. The feature flag should not be necessary, but because the argparse library is so flexible, it better to be safe than sorry.

• Issue: Argparse: Cryptic usage message when combining choices with type #132558

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[StanFromIreland]

You also need to...

• Update docs
• Add a NEWS entry
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[hansthen]

You also need to...

• Update docs
• Add a NEWS entry
• Sign the CLA

Done, thanks for the help!

[StanFromIreland]

So it was never a bug ;-)

[hansthen]

Okay, you got me. I'm convinced now :-)

[StanFromIreland]

Talk about the new option, keep it brief. Eg. Added X to :mod:... which allows ... .

[savannahostrowski]

Hey there - thanks for jumping in on a PR. However, as mentioned on the issue, maybe we can start by clarifying the documentation. I’d prefer not to expand the API surface area by strapping another parameter onto ArgumentParser. This is a pattern we need to be really intentional about.