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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def unsubscribe(self, topic):
 # type: (str) -> Optional[Tuple[int, int]] """ Unsubscribe from a single topic. :param topic: a single string that is the subscription topic to unsubscribe from :rtype: (int, int) :result: (result, mid) Returns a tuple (result, mid), where result is MQTT_ERR_SUCCESS to indicate success or (MQTT_ERR_NO_CONN, None) if the client is not currently connected. mid is the message ID for the unsubscribe request. The mid value can be used to track the unsubscribe request by checking against the mid argument in the on_unsubscribe() callback if it is defined. """

        # don't unsubscribe if not in topics
        if topic in self.topics:
            result, mid = self.client.unsubscribe(topic)

            if result == MQTT_ERR_SUCCESS:
                self.topics.pop(topic)
                logger.debug('Unsubscribed from topic: {0}'.format(topic))
            else:
                logger.debug('Error {0} unsubscribing from topic: {1}'
                             .format(result, topic))

            # if successful remove from topics
            return result, mid
        return None