def get_argument_role(self):
try:<EOL><INDENT>return self.get_argument(constants.PARAM_ROLE, default=None)<EOL><DEDENT>except tornado.web.MissingArgumentError as e:<EOL><INDENT>raise Exception(e.log_message)<EOL><DEDENT>
Helper function to get request argument.
Raises exception if argument is missing.
Returns the role argument.