In Python argparse, is it possible to have paired –no-something/–something arguments?

Well, none of the answers so far are quite satisfactory for a variety of reasons. So here is my own answer:

class ActionNoYes(argparse.Action):
    def __init__(self, opt_name, dest, default=True, required=False, help=None):
        super(ActionNoYes, self).__init__(['--' + opt_name, '--no-' + opt_name], dest, nargs=0, const=None, default=default, required=required, help=help)
    def __call__(self, parser, namespace, values, option_string=None):
        if option_string.starts_with('--no-'):
            setattr(namespace, self.dest, False)
        else:
            setattr(namespace, self.dest, True)

And an example of use:

>>> p = argparse.ArgumentParser()
>>> p._add_action(ActionNoYes('foo', 'foo', help="Do (or do not) foo. (default do)"))
ActionNoYes(option_strings=['--foo', '--no-foo'], dest="foo", nargs=0, const=None, default=True, type=None, choices=None, help='Do (or do not) foo. (default do)', metavar=None)
>>> p.parse_args(['--no-foo', '--foo', '--no-foo'])
Namespace(foo=False)
>>> p.print_help()
usage: -c [-h] [--foo]

optional arguments:
  -h, --help       show this help message and exit
  --foo, --no-foo  Do (or do not) foo. (default do)

Unfortunately, the _add_action member function isn’t documented, so this isn’t ‘official’ in terms of being supported by the API. Also, Action is mainly a holder class. It has very little behavior on its own. It would be nice if it were possible to use it to customize the help message a bit more. For example saying --[no-]foo at the beginning. But that part is auto-generated by stuff outside the Action class.