上面代码，不使用多线程，而是改成在两个方法中订阅不同的topic

把代码分成两个方法，分别订阅不同的topic：

import paho.mqtt.client as mqtt

def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))
    client.subscribe("topic1")

def on_message1(client, userdata, msg):
    print(msg.topic+" "+str(msg.payload))

def on_message2(client, userdata, msg):
    print(msg.topic+" "+str(msg.payload))

client1 = mqtt.Client()
client1.on_connect = on_connect
client1.on_message = on_message1
client1.connect("localhost", 1883, 60)

client2 = mqtt.Client()
client2.on_connect = on_connect
client2.on_message = on_message2
client2.connect("localhost", 1883, 60)

client1.loop_forever()
client2.loop_forever()

其中，`on_message1`方法订阅了`topic1`，`on_message2`方法可以订阅其他topic。在创建`client1`和`client2`时，分别将不同的方法分配给`on_message`属性。最后，分别调用两个客户端的`loop_forever`方法，以保持连接并处理消息。