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.