def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def add_arguments(self, parser):
        parser.add_argument('--noinput', action='store_false',
                            dest='interactive', default=True,
                            help='Do not prompt for input of any kind.')
        parser.add_argument('-l', '--list', action='store_true',
                            dest='list', default=False,
                            help="Just list the requirements, don't install them.")