def get_argument_topology(self):
try:<EOL><INDENT>topology = self.get_argument(constants.PARAM_TOPOLOGY)<EOL>return topology<EOL><DEDENT>except tornado.web.MissingArgumentError as e:<EOL><INDENT>raise Exception(e.log_message)<EOL><DEDENT>
Helper function to get topology argument.
Raises exception if argument is missing.
Returns the topology argument.