argparse incorrectly says arguments are required

[eslindsey]

When using nargs='*' with other required arguments, argparse incorrectly says the optional argument is required.

Output (with no arguments)

usage: example [-h] foo [bar ...]
example: error: the following arguments are required: foo, bar

Code

import argparse

parser = argparse.ArgumentParser(prog='example')
parser.add_argument('foo')
parser.add_argument('bar', nargs='*')
args = parser.parse_args()

Python 3.9.16

Linked PRs

• GH-103219: Fix optional args incorrectly being processed as required args #123931

[ericvsmith]

You'll need to show your code, otherwise we can't tell if this is a bug in argparse or your code. Please create a minimal reproducible example: https://stackoverflow.com/help/minimal-reproducible-example

[eslindsey]

Added a minimal reproducible example.

[sobolevn]

The patch can look something like this:

                if (action.required and
                    action.default is None and
                    action.nargs not in (ZERO_OR_MORE, OPTIONAL)):
                    required_actions.append(_get_action_name(action))

On line

cpython/Lib/argparse.py

Line 2127 in 24facd6

if action.required:

Test case:

    def test_required_args(self):  # gh-103219
        parser = ErrorRaisingArgumentParser(prog='example')
        parser.add_argument('foo_arg')
        parser.add_argument('baz_arg', default='baz')
        parser.add_argument('bar_arg', nargs='*')

        args = parser.parse_args(['a'])
        self.assertEqual(
            NS(foo_arg='a', baz_arg='baz', bar_arg=[]), args)

        args = parser.parse_args(['a', 'b', 'c', 'd'])
        self.assertEqual(
            NS(foo_arg='a', baz_arg='b', bar_arg=['c', 'd']), args)

        with self.assertRaises(ArgumentParserError) as cm:
            parser.parse_args([])
        self.assertRegex(
            cm.exception.stderr,
            'error: the following arguments are required: foo_arg\n$'
        )

But, I don't feel confident submitting argparse patches. I hope this will help someone else :)

[hpaulj]

I don't think this needs to be fixed. It's an attempt to correct minor issue in the error message.

Normally '*' and '?' positionals are 'seen', because an empty list of arguments meets their nargs requirments.

In this case required 'foo' positional, blocks all further processing of positionals, including 'bar'. 'bar' can't be filled, with default or not, until 'foo' has been filled.

[sobolevn]

I don't think this needs to be fixed. It's an attempt to correct minor issue in the error message.

Why don't we need to fix an error message?

[serhiy-storchaka]

This is a duplicate of #72795.

[ericvsmith]

Since it's a duplicate, I'm closing this.