I was referring to the fact that optionals have an additional case that positionals don't have: "Note that for optional arguments, there is an additional case -- the option string is present but not followed by a command-line argument."

(from http://docs.python.org/dev/library/argparse.html#nargs )

>>> p.add_argument('--foo', nargs='*', default=None)
>>> p.parse_args([])
Namespace(foo=None)
>>> p.parse_args(['--foo'])
Namespace(foo=[])

So it could be argued that positionals (at least by default) are behaving like the second case.  But that's as far as the parallel goes apparently.  *default* affects the first case and not the second case for optional arguments:

>>> p.add_argument('--foo', nargs='*', default=False)
>>> p.parse_args([])
Namespace(foo=False)
>>> p.parse_args(['--foo'])
Namespace(foo=[])