def subscribe(self, topic, qos=0):
        # type: (str, int) -> Tuple[int, int]
        """
        Subscribe to a certain topic.

        :param topic: a string specifying the subscription topic to
            subscribe to.
        :param qos: the desired quality of service level for the subscription.
                    Defaults to 0.

        :rtype: (int, int)
        :result: (result, mid)

        A topic is a UTF-8 string, which is used by the broker to filter
        messages for each connected client. A topic consists of one or more
        topic levels. Each topic level is separated by a forward slash
        (topic level separator).

        The function 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
        subscribe request. The mid value can be used to track the subscribe
        request by checking against the mid argument in the on_subscribe()
        callback if it is defined.

        **Topic example:** `myhome/groundfloor/livingroom/temperature`

        """
        # TODO: add support for list of topics

        # don't subscribe if already subscribed
        # try to subscribe
        result, mid = self.client.subscribe(topic=topic, qos=qos)

        # if successful add to topics
        if result == MQTT_ERR_SUCCESS:
            self.topics[topic] = TopicQos(topic=topic, qos=qos)
            logger.debug('Subscribed to topic: {0}, qos: {1}'
                         .format(topic, qos))
        else:
            logger.error('Error {0} subscribing to topic: {1}'
                         .format(result, topic))

        return (result, mid)
Subscribe to a certain topic.

        :param topic: a string specifying the subscription topic to
            subscribe to.
        :param qos: the desired quality of service level for the subscription.
                    Defaults to 0.

        :rtype: (int, int)
        :result: (result, mid)

        A topic is a UTF-8 string, which is used by the broker to filter
        messages for each connected client. A topic consists of one or more
        topic levels. Each topic level is separated by a forward slash
        (topic level separator).

        The function 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
        subscribe request. The mid value can be used to track the subscribe
        request by checking against the mid argument in the on_subscribe()
        callback if it is defined.

        **Topic example:** `myhome/groundfloor/livingroom/temperature`