无法为树莓派将MQTT分配给python中的变量

Question

我正在尝试将传入的MQTT消息分配给一个变量。该变量需要在方法外部可以访问，但是，我无法实现。 当我发送MQTT消息时，没有任何反应。

没有错误消息

import paho.mqtt.client as mqtt #import the client1
import time

class Mqttreceiving:
    def mqttreceivingMethod():
        global mqttMesssage
        def on_message(client, userdata, message):                
             mqttMesssage = str(message.payload.decode("utf-8"))

             #should return mqtt value outside method
             return mqttMesssage       

    ########################################
    broker_address="192.168.0.31"
    client = mqtt.Client("Secondpi") #create new instance
    client.connect(broker_address) #connect to broker
    print("Waiting to receive mqtt message")
    client.on_message=on_message 
    client.loop_start()
    client.subscribe("carpark/message")
    time.sleep(4) # wait
    client.loop_stop() # stop the loop


#should return back value of mqtt
Mqttreceiving.mqttreceivingMethod()