The code calls the client.subscribe method with "test/drone01" as an argument to subscribe to this specific single topic and the qos argument set to 2 to request a QoS level of 2.
Tip
In this case, we will only subscribe to one topic. However, it is very important to know that we are not limited to subscribe to a single topic filter; we may subscribe to many topic filters with a single call to the subscribe method.
After the MQTT server confirms the successful subscription to the specified topic filter, the specified callback in the client.on_subscribe attribute will be executed, that is, the on_subscribe function. This function receives a list of integers in the granted_qos argument that provides the QoS level that the MQTT server has granted for each of the topic filter subscription requests. The code in the on_subscribe function displays the QoS level granted by the MQTT server for the topic filter we specified.
Whenever there is a new message received that...
