The code calls the client.subscribe method with "vehicles/vehiclepi01/tests" as an argument to subscribe to this specific single topic and the qos argument set to 2 to request a QoS level of 2.
In this case, we will only subscribe to one topic. However, it is very important to know that we are not limited to subscribing to a single topic filter; we might 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 with a SUBACK response, 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....