def __init__(self, username, key, service_host='io.adafruit.com', service_port=1883):
        self._username = username
        self._service_host = service_host
        self._service_port = service_port
        # Initialize event callbacks to be None so they don't fire.
        self.on_connect    = None
        self.on_disconnect = None
        self.on_message    = None
        # Initialize MQTT client.
        self._client = mqtt.Client()
        self._client.username_pw_set(username, key)
        self._client.on_connect    = self._mqtt_connect
        self._client.on_disconnect = self._mqtt_disconnect
        self._client.on_message    = self._mqtt_message
        self._connected = False
Create instance of MQTT client.
dataset/ETHPy150Open adafruit/io-client-python/Adafruit_IO/mqtt_client.py/MQTTClient.__init__