Here's four commands to demonstrate my problem:

My sample code:
$ cat prog.py 
import argparse
parse = argparse.ArgumentParser()
parse.add_argument("foo", nargs=2, metavar=("foo", "bar"))
args = parse.parse_args()
print(args)

This output could be friendlier:
$ python3 prog.py --help
Traceback (most recent call last):
  File "prog.py", line 4, in <module>
    args = parse.parse_args()
  File "/home/wena/cpython/Lib/argparse.py", line 1722, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/home/wena/cpython/Lib/argparse.py", line 1754, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/home/wena/cpython/Lib/argparse.py", line 1960, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/home/wena/cpython/Lib/argparse.py", line 1900, in consume_optional
    take_action(action, args, option_string)
  File "/home/wena/cpython/Lib/argparse.py", line 1828, in take_action
    action(self, namespace, argument_values, option_string)
  File "/home/wena/cpython/Lib/argparse.py", line 1015, in __call__
    parser.print_help()
  File "/home/wena/cpython/Lib/argparse.py", line 2347, in print_help
    self._print_message(self.format_help(), file)
  File "/home/wena/cpython/Lib/argparse.py", line 2314, in format_help
    formatter.add_arguments(action_group._group_actions)
  File "/home/wena/cpython/Lib/argparse.py", line 270, in add_arguments
    self.add_argument(action)
  File "/home/wena/cpython/Lib/argparse.py", line 255, in add_argument
    invocations = [get_invocation(action)]
  File "/home/wena/cpython/Lib/argparse.py", line 533, in _format_action_invocation
    metavar, = self._metavar_formatter(action, default)(1)
ValueError: too many values to unpack (expected 1)

On Python 3.2 (from Debian), I get "error: too few arguments". But with latest mercurial I get:
$ python3 prog.py
Traceback (most recent call last):
  File "prog.py", line 4, in <module>
    args = parse.parse_args()
  File "/home/wena/cpython/Lib/argparse.py", line 1722, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/home/wena/cpython/Lib/argparse.py", line 1754, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/home/wena/cpython/Lib/argparse.py", line 1973, in _parse_known_args
    ', '.join(required_actions))
TypeError: sequence item 0: expected str instance, tuple found

But this looks okay:
$ python3 prog.py a b
Namespace(foo=['a', 'b'])

Looks like the problem is that "_format_action_invocation" is not being as careful with the different possibilities for metavar as "_format_args" is.  Patches welcome!

This patch fixes the problem by joining the metavar terms with '|'.  So the help for the test case (adapted from an existing tuple test) looks like:

    usage: PROG [-h] W1 [W2 ...] [X1 [X2 ...]] Y1 Y2 Y3 [Z1]
    positional arguments:
      W1|W2       w
      X1|X2       x
      Y1|Y2|Y3    y
      Z1          z

Alternatives include:
- use ',',' ',or '/' instead of '|'
- use just the 1st item of the tuple
- use the default (non-tuple) metavar in the help line
These all pass existing tests.

The last alternative would use:

    #metavar = '|'.join(metavar)
    if len(metavar)>1:
        metavar = default
    else:
        metavar = metavar[0]

oops - to fix the error message that OP complained about, I need to patch '_get_action_name' as well:

    def _get_action_name(argument):
    ...
        elif argument.metavar not in (None, SUPPRESS):
            metavar = argument.metavar
            if isinstance(metavar, tuple):
                metavar = '|'.join(metavar)
            return metavar

This patch fixes both help and error formatting.

A module level '_format_metavars' does the formatting for both.

I have tried several alternatives, including using the 'usage' style.

There is similarity between this fix and that for issue 16468 (custom choices), though I don't think they conflict.  In both cases, code needed to format the usage or help is also needed to help format error messages.

Issue 9849 (better nargs warning) is another case where error checking in the parser depends on the formatter.  In the long run we may want to refactor these issues.

An alternative fix is to disallow tuple metavars for positionals.  

A tuple metavar may look nice in the usage.  But a well selected dest seems better in the help line, and error messages.  Using dest in all 3 roles, as we do now, is the simplest compromise.

Can confirm this bug still exists on master branch, python3.7, python3.6, and very likely other versions since it's reported.

It seems only `_format_action_invocation` and `_get_action_name` need to be fixed. So we can do it more lightweight (<10 lines).

-0 on going forward with this.  AFAICT, no one has requested tuple support (the OP just wanted better error handling).  And more fined grained control can already be had just by specifying separate positional arguments.

Paul suggested that it may be a better plan to just disallow tuple metavars and to give a better error message as the OP suggested.  That makes sense to me.

I am not sure disallowing tuple metavars would break backup compatibility or not~

> I am not sure disallowing tuple metavars would break backup 
> compatibility or not~

Given that tuple metavars currently doesn't work, there is no backward compatibility issue.  What is being proposed is to give more friendly error messages.

> Given that tuple metavars currently doesn't work, there is no backward compatibility issue.  What is being proposed is to give more friendly error messages.

Got it, thanks.

> Paul suggested that it may be a better plan to just disallow tuple metavars and to give a better error message as the OP suggested.  That makes sense to me.

+1

Tuple support is documented:

https://github.com/python/cpython/commit/98047eb806227f11212f6a42c242030a51964e30#diff-9c4a053d29149ba40370fbdddd3e34faR1059

https://docs.python.org/3/library/argparse.html#metavar

>   Providing a tuple to ``metavar`` specifies a different display for each of the arguments:

From his word here I feel like our professor meant to support tuple metavar but the implementation was not careful. He called for a patch eight years ago. Let's summon him and see if he has changed his mind?

Reproduced on 3.11, with a different error:

% ./python.exe prog.py
Traceback (most recent call last):
  File "/Users/iritkatriel/src/cpython-1/prog.py", line 5, in <module>
    args = parse.parse_args()
           ^^^^^^^^^^^^^^^^^^
  File "/Users/iritkatriel/src/cpython-1/Lib/argparse.py", line 1822, in parse_args
    args, argv = self.parse_known_args(args, namespace)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/iritkatriel/src/cpython-1/Lib/argparse.py", line 1855, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/iritkatriel/src/cpython-1/Lib/argparse.py", line 2093, in _parse_known_args
    ', '.join(required_actions))
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: sequence item 0: expected str instance, tuple found