[From http://code.google.com/p/argparse/issues/detail?id=61]

It should be documented clearly that only the arguments present on the parent parser at the time ArgumentParser is called will be included in the parser.

>>> parent = argparse.ArgumentParser(add_help=False)
>>> child = argparse.ArgumentParser(parents=[parent])
>>> parent.add_argument('--foo', action='store_true')
>>> child.parse_args(['--foo'])
usage: [-h]
: error: unrecognized arguments: --foo