Return an argparse.Namespace of the argv string or sys.argv if
        argv is None.

def parse_args(self, argv=None):
        """ Return an argparse.Namespace of the argv string or sys.argv if
        argv is None. """
        arg_input = shlex.split(argv) if argv is not None else None
        self.get_or_create_session()
        return self.argparser.parse_args(arg_input)