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>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', nargs='a')
 ...
 File ".../Lib/argparse.py", line 1333, in add_argument
 raise ValueError("length of metavar tuple does not match nargs")
ValueError: length of metavar tuple does not match nargs
The message should really be about nargs not having a valid value. The nargs value is invalid regardless of the metavar. There is also this:
>>> 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)
which is not consistent with this:
>>> parser.add_argument('foo', nargs=0)
 ...
 raise ValueError('nargs for store actions must be > 0; if you '
ValueError: nargs for store actions must be > 0; if you have nothing to store, actions such as store true or store const may be more appropriate