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

[321b01e9-392b-44d4-bb60-82ff20cce040]

BPO	42547
Nosy	@rhettinger, @iritkatriel, @m-khvoinitsky
PRs	python/cpython#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: ```python assignee = None closed_at = None created_at = labels = ['type-bug', 'library', '3.10'] title = "argparse: add_argument(... nargs='+', metavar=) does not work with positional arguments" updated_at = user = 'https://github.com/m-khvoinitsky' ``` bugs.python.org fields: ```python activity = actor = 'iritkatriel' assignee = 'none' closed = False closed_date = None closer = None components = ['Library (Lib)'] creation = 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'] ```

[321b01e9-392b-44d4-bb60-82ff20cce040]

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

[7a064fe6-c535-4d80-a11f-a04ed39056c5]

Duplicate of

https://bugs.python.org/issue14074

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

[7a064fe6-c535-4d80-a11f-a04ed39056c5]

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.

[serhiy-storchaka]

124782 is more complete solution of this issue.