Adds an insufficient_data action, represented as an SNS topic, to
        this alarm. What to do when the insufficient_data state is reached.

        :type action_arn: str
        :param action_arn: SNS topics to which notification should be 
                           sent if the alarm goes to state INSUFFICIENT_DATA.

def add_insufficient_data_action(self, action_arn=None):
        """
        Adds an insufficient_data action, represented as an SNS topic, to
        this alarm. What to do when the insufficient_data state is reached.

        :type action_arn: str
        :param action_arn: SNS topics to which notification should be 
                           sent if the alarm goes to state INSUFFICIENT_DATA.
        """
        if not action_arn:
            return
        self.actions_enabled = 'true'
        self.insufficient_data_actions.append(action_arn)