Global variables

The BROKER_HOST and BROKER_POST variables at line (2) are referring to our locally installed Mosquitto MQTT broker. Port 1883 is the standard default MQTT port:

# Global Variables
...
BROKER_HOST = "localhost"   # (2)
BROKER_PORT = 1883
CLIENT_ID = "LEDClient"     # (3)
TOPIC = "led"               # (4)
client = None # MQTT client instance. See init_mqtt()   # (5)
...

At line (3), we define CLIENT_ID, which will be the unique client identifier we use to identify our program with the Mosquitto MQTT broker. We must provide a unique ID to the broker so that we can use durable connections.

At line (4), we define the MQTT topic that our program will be subscribing to, while at line (5), the client variable is a placeholder that will be assigned the Paho-MQTT client instance, which we'll see shortly.