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argparse fails to parse [] when using choices and nargs='*' #71414

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evanunderscore mannequin opened this issue Jun 5, 2016 · 4 comments
Open

argparse fails to parse [] when using choices and nargs='*' #71414

evanunderscore mannequin opened this issue Jun 5, 2016 · 4 comments
Labels
3.9 only security fixes 3.10 only security fixes 3.11 only security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error

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@evanunderscore
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evanunderscore mannequin commented Jun 5, 2016

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
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    evanunderscore mannequin commented Jun 5, 2016

    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.)

    @evanunderscore evanunderscore mannequin added stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error labels Jun 5, 2016
    @berkerpeksag
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    Looking at this again, I think we should fix this in bpo-9625.

    @macfreek
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    macfreek mannequin commented Apr 26, 2018

    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.

    @macfreek macfreek mannequin added the 3.7 (EOL) end of life label Apr 26, 2018
    @paulj3 paulj3 mannequin reopened this Aug 31, 2019
    @iritkatriel
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    Reproduced on 3.11.

    @iritkatriel iritkatriel added 3.9 only security fixes 3.10 only security fixes 3.11 only security fixes and removed 3.7 (EOL) end of life labels Dec 11, 2021
    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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    3.9 only security fixes 3.10 only security fixes 3.11 only security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error
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