argparse: add_argument(... nargs='+', metavar=<tuple>) does not work with positional arguments

[m-khvoinitsky]

BPO	42547
Nosy	@rhettinger, @iritkatriel, @m-khvoinitsky
PRs	bpo-42547: fix add_argument(metavar=<tuple>, nargs='+') for positional arguments #23617

^{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 2020-12-02.21:15:50.599>
labels = ['type-bug', 'library', '3.10']
title = "argparse: add_argument(... nargs='+', metavar=<tuple>) does not work with positional arguments"
updated_at = <Date 2021-09-07.19:41:21.754>
user = 'https://github.com/m-khvoinitsky'

bugs.python.org fields:

activity = <Date 2021-09-07.19:41:21.754>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-12-02.21:15:50.599>
creator = 'm_khvoinitsky'
dependencies = []
files = []
hgrepos = []
issue_num = 42547
keywords = ['patch']
message_count = 4.0
messages = ['382341', '382368', '382369', '401340']
nosy_count = 4.0
nosy_names = ['rhettinger', 'paul.j3', 'iritkatriel', 'm_khvoinitsky']
pr_nums = ['23617']
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42547'
versions = ['Python 3.10']

[m-khvoinitsky]

Example which works:

    parser.add_argument('--test', nargs='+', metavar=('TEST', 'TEST2'))

Example which doesn't work:

    parser.add_argument('test', nargs='+', metavar=('TEST', 'TEST2'))

it raises:

    Traceback (most recent call last):
      File <omitted>
        args = parser.parse_args()
      File "/usr/lib/python3.8/argparse.py", line 1768, in parse_args
        args, argv = self.parse_known_args(args, namespace)
      File "/usr/lib/python3.8/argparse.py", line 1800, in parse_known_args
        namespace, args = self._parse_known_args(args, namespace)
      File "/usr/lib/python3.8/argparse.py", line 2006, in _parse_known_args
        start_index = consume_optional(start_index)
      File "/usr/lib/python3.8/argparse.py", line 1946, in consume_optional
        take_action(action, args, option_string)
      File "/usr/lib/python3.8/argparse.py", line 1874, in take_action
        action(self, namespace, argument_values, option_string)
      File "/usr/lib/python3.8/argparse.py", line 1044, in __call__
        parser.print_help()
      File "/usr/lib/python3.8/argparse.py", line 2494, in print_help
        self._print_message(self.format_help(), file)
      File "/usr/lib/python3.8/argparse.py", line 2471, in format_help
        formatter.add_arguments(action_group._group_actions)
      File "/usr/lib/python3.8/argparse.py", line 276, in add_arguments
        self.add_argument(action)
      File "/usr/lib/python3.8/argparse.py", line 261, in add_argument
        invocations = [get_invocation(action)]
      File "/usr/lib/python3.8/argparse.py", line 549, in _format_action_invocation
        metavar, = self._metavar_formatter(action, default)(1)
    ValueError: too many values to unpack (expected 1)

Expected result: help message should look like this:

usage: test_argparse [-h] TEST [TEST2 ...]

positional arguments:
  TEST

optional arguments:
  -h, --help  show this help message and exit

[paulj3]

Duplicate of

https://bugs.python.org/issue14074

argparse allows nargs>1 for positional arguments but doesn't allow metavar to be a tuple

[paulj3]

I'll reopen it - your patch, while not a complete resolution, does take care of the immediate error.

[iritkatriel]

Changing type since crash typically means segfault and not an exception.