argparse fails to parse [] when using choices and nargs='*'

[evanunderscore]

BPO	27227
Nosy	@macfreek, @berkerpeksag, @evanunderscore, @iritkatriel
Superseder	bpo-9625: argparse: Problem with defaults for variable nargs when using choices

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-06-05.08:59:16.765>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = "argparse fails to parse [] when using choices and nargs='*'"
updated_at = <Date 2021-12-11.18:35:18.463>
user = 'https://github.com/evanunderscore'

bugs.python.org fields:

activity = <Date 2021-12-11.18:35:18.463>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-06-05.08:59:16.765>
creator = 'evan_'
dependencies = []
files = []
hgrepos = []
issue_num = 27227
keywords = []
message_count = 4.0
messages = ['267404', '268387', '315783', '408330']
nosy_count = 5.0
nosy_names = ['macfreek', 'berker.peksag', 'paul.j3', 'evan_', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'open'
superseder = '9625'
type = 'behavior'
url = 'https://bugs.python.org/issue27227'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[evanunderscore]

When using nargs='*' with choices, it is impossible to specify 0 args:

from argparse import ArgumentParser
parser = ArgumentParser()
parser.add_argument('foo', choices=['foo'], nargs='*')
args = parser.parse_args([])  # <-- fails, error message below
assert args.foo == []
# usage: args.py [-h] [{foo} [{foo} ...]]
# args.py: error: argument foo: invalid choice: [] (choose from 'foo')

The problem appears to be this block of code trying to validate value immediately after it sets it to []:

    # when nargs='*' on a positional, if there were no command-line
    # args, use the default if it is anything other than None
    elif (not arg_strings and action.nargs == ZERO_OR_MORE and
          not action.option_strings):
        if action.default is not None:
            value = action.default
        else:
            value = arg_strings
        self._check_value(action, value)

The fix seems to be as simple as moving the check under if action.default is not None.

(NOTE: This would be also adequately solved by patches already attached to http://bugs.python.org/issue9625, however the minimal solution to this problem is simpler.)

[berkerpeksag]

Looking at this again, I think we should fix this in bpo-9625.

[macfreek]

I don't think that this bpo-27227 and bpo-9625 are the same (although they may have the same root cause, and I would be in favour of fixing both issues). At least, I think the unit test are distinct:

This bpo-27227 is that argparse does not accept 0 options is '0 or more' are specified in nargs. bpo-9625 is that argparse returns a different type if a default is specified or when it is not.

So I propose to reopen this ticket: it is not fixed, and different from the referred ticket.

[iritkatriel]

Reproduced on 3.11.

[savannahostrowski]

This is a duplicate of #92445, which was fixed with ad7340e.