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On a related point, the 'action.required' value is set differently for '?' and '*' positionals. 
 if kwargs.get('nargs') not in [OPTIONAL, ZERO_OR_MORE]:
 kwargs['required'] = True
 if kwargs.get('nargs') == ZERO_OR_MORE and 'default' not in kwargs:
 kwargs['required'] = True
OPTIONAL is always not required, ZERO_OR_MORE is not required if it has a default. But for reasons discussed here, that 'required' value makes little difference. 
`parse_args` checks that all 'required' arguments have been seen, but a ZERO_OR_MORE positional is always seen (i.e. it matches an empty string). 
Usage formatting always uses '[%s [%s ...]]' with ZERO_OR_MORE, regardless of the 'required' attribute.
The only place where this 'required' value seems to matter is when adding such an argument to a mutually exclusive group. But if an unused '*' positional is going to get a '[]' value anyways, why should it be excluded from such a use?
If I remove the 
 if kwargs.get('nargs') == ZERO_OR_MORE and 'default' not in kwargs:
test, test_argparse.py still runs fine.
http://bugs.python.org/issue18943 is a possibly related issue, involving a 'is not action.default' test in a mutually exclusive group.