Thanks for Hint !
Do you know how to avoid this outputs in Session Log :
pi@raspberrypi:~/Luminosity $ python light_sensor_2.py
Connecting to mqtt.mydevices.com:1883
Connected with result code 0
SUB v1/REMOVED/things/REMOVED/cmd/+
PUB v1/REMOVED/things/REMOVED/sys/model Python
PUB v1/REMOVED/things/REMOVED/sys/version 1.1.0
PUB v1/REMOVED/things/REMOVED/data/2 181.666666667
PUB v1/REMOVED/things/REMOVED/data/2 184
PUB v1/REMOVED/things/REMOVED/data/2 185