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