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This is referring to argparse 1.1 installed under Python 2.6. When I was passing in an nargs flag, I figured that since '+' and '*' are valid options, I should pass in strings. So when I tried passing in the string '1' instead of the integer 1, I got the following error:
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', nargs='1')
_StoreAction(option_strings=[], dest='foo', nargs='1', const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>> parser.parse_args()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "build/bdist.macosx-10.6-universal/egg/argparse.py", line 1698, in parse_args
 File "build/bdist.macosx-10.6-universal/egg/argparse.py", line 1730, in parse_known_args
 File "build/bdist.macosx-10.6-universal/egg/argparse.py", line 1935, in _parse_known_args
 File "build/bdist.macosx-10.6-universal/egg/argparse.py", line 1884, in consume_positionals
 File "build/bdist.macosx-10.6-universal/egg/argparse.py", line 2028, in _match_arguments_partial
 File "build/bdist.macosx-10.6-universal/egg/argparse.py", line 2169, in _get_nargs_pattern
TypeError: can't multiply sequence by non-int of type 'str'
Fortunately, I had just added the nargs and knew to correct that. However, if I were to do something like read that value in from a config file and forget to coerce the value from a string to an int, I could see how this could be a giant pain to track down.

Note, argparse is not part of the Python standard library in 2.6 but the 2.7 and 3.2 versions exhibit the same behavior.

The behaviour has changed from that given in msg116355. Using Python 3.3.1 on Windows.
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', nargs='1')
Traceback (most recent call last):
 File "c:\python33\lib\argparse.py", line 1322, in add_argument
 self._get_formatter()._format_args(action, None)
 File "c:\python33\lib\argparse.py", line 585, in _format_args
 formats = ['%s' for _ in range(action.nargs)]
TypeError: 'str' object cannot be interpreted as an integer
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "c:\python33\lib\argparse.py", line 1324, in add_argument
 raise ValueError("length of metavar tuple does not match nargs")
ValueError: length of metavar tuple does not match nargs
The docs http://docs.python.org/3/library/argparse.html#nargs are clear that nargs is an integer or various types of string so I think this could be closed as already fixed.

It does shift the error from parse_args to add_argument, but the message 'ValueError: length of metavar tuple does not match nargs', indicates that it's a side effect of checking on the tuple form of `metavar`.
http://bugs.python.org/issue9348
There is still room for cleaning up these tests. There are 2 functions that define what are acceptable values for nargs, [None, OPTIONAL, ZERO_OR_MORE, ONE_OR_MORE, REMAINDER, PARSER, integer]. Should one or other explicitly check nargs is an integer if it does not match one of the other strings?
And the test in _ActionContainer.add_argument()
 # raise an error if the metavar does not match the type
 if hasattr(self, "_get_formatter"):
 try:
 self._get_formatter()._format_args(action, None)
 except TypeError:
 raise ValueError("length of metavar tuple does not match nargs")
uses an ArgumentParser._get_formatter method (though 'hasattr' prevents runtime errors). In fact both functions that use nargs belong to the parser, not the container. I wonder if `add_argument` should be moved to ArgumentParser.

The first error raised is "TypeError: 'str' object cannot be interpreted as an integer", followed by "ValueError: length of metavar tuple does not match nargs". Therefore the code has already been changed to reflect the title of this issue. If other code changes are needed I believe that should be done in a new or another already existing issue.

This nargs test using the formater applies only when the container has a help formatter. That is true for a ArgumentParser, but not for an argument_group.
group = parser.add_argument_group('g')
group.add_argument('bar', nargs='test')
does not raise an error.
format_help will produce an error: ...
 File "./argparse.py", line 585, in _format_args
 formats = ['%s' for _ in range(action.nargs)]
 TypeError: 'str' object cannot be interpreted as an integer
while parse_args produces the error: ...
 File "./argparse.py", line 2200, in _get_nargs_pattern
 nargs_pattern = '(-*%s-*)' % '-*'.join('A' * nargs)
 TypeError: can't multiply sequence by non-int of type 'str'

This patch adds a value test for nargs during add_argument. The core of the new test is in ArgumentParser._check_argument.
add_argument() now calls ArgumentParser._check_argument(), which calls _format_args(). If it gets a TypeError, it raises a metavar ValueError as before. But if it gets an ValueError, it raises an ArgumentError.
An argument group gets the _check_argument method from its container. This way, check_argument() works for both parsers and groups.
HelpFormater _format_args() now raises a ValueError if the nargs is not an integer (or one of the recognized strings). 
What kind of error should we produce when there is a problem with nargs?
An ArgumentError has the advantage that it includes the action name. But the metavar tuple test was coded to raise ValueError. Plus the test_argparse.py TestInvalidArgumentConstructors class tests all check for TypeError or ValueError.
I have kept the metavar tuple case as ValueError. That way, test_argparse.py runs without a change. I still need to add tests for invalid string nargs values. And I think the case could be made for returning ArgumentValue errors.

The attached test_nargswarn.py file tests the argparse.py patch. It tries out various nargs values, both for parsers, groups, and mutually exclusive groups. I intend to recast these to work in test_argparse.py

This is a revision of yesterday's patch. It includes a couple of test cases that check that parser, groups, and exclusive groups all produce the error message.
I also changed the metavar tuple case to return an ArgumentError, and changed test_argparse.py accordingly. (this metavar testing was added in http://bugs.python.org/issue9348).
This issue overlaps with http://bugs.python.org/issue16970, 'argparse: bad nargs value raises misleading message'. There, though, the focus is more on the error message when nargs=0. While nargs<1 might not make much sense, it technically does not break anything.

I included this patch (with minor changes) in a patch that I just posted to http://bugs.python.org/issue16468. That issue deals with the argument choices option, which can be tested along with nargs and metavars.

http://bugs.python.org/issue21666
raises the possibility of testing the 'help' parameter in the same way. By adding (to _check_argument):
 # check the 'help' string
 try:
 self._get_formatter()._expand_help(action)
 except (ValueError, TypeError, KeyError) as e:
 raise ArgumentError(action, 'badly formed help string')

This is fixed now:
Python 3.10.0a2+ (heads/bpo-42184:f3cb814315, Nov 7 2020, 00:31:51) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', nargs='1')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "C:\Users\User\src\cpython\lib\argparse.py", line 1430, in add_argument
 self._get_formatter()._format_args(action, None)
 File "C:\Users\User\src\cpython\lib\argparse.py", line 611, in _format_args
 raise ValueError("invalid nargs value") from None
ValueError: invalid nargs value