无需订阅 python Paho MQTT 库即可接收消息

Question

我正在做一个项目，我需要两台计算机通过 MQTT 进行通信。为了测试它，我创建了这段代码。我运行我笔记本电脑上的这段代码：

import paho.mqtt.client as mqtt
import time
def on_connect(client,userdata,flags,rc):
    if rc == 0:
        print("connected OK")
    else:
        print("bad connection Returned code=",rc)
    client.subscribe('test/message')
def on_subscribe():
    print('hello')
def on_message(client,userdata,msg):
    message=str(msg.payload.decode())
    print(message)
broker = "10.49.12.253"
client = mqtt.Client("Get")
client.on_subscribe = on_subscribe
client.on_connect = on_connect
client.on_message = on_message
print("Connecting to broker ",broker)
client.connect(broker)
client.loop_forever()

从另一台计算机发布消息后，我得到了这个输出：

Connecting to broker  10.49.12.253
connected OK
Hello, world

我发送消息的计算机也是代理。我很困惑，因为 on_subscribe 函数从未被调用过，我不明白为什么我仍然收到消息。这是怎么回事？

Answer 1

问题是您的 on_subscribe() 函数缺少所需数量的参数。应该是：

...
def on_subscribe(client, userdata, mid, granted_qos):
  print("hello")
...

Paho 在 try/except 块中触发回调，并吞下错误签名会抛出的错误消息。

文档：https://www.eclipse.org/paho/index.php?page=clients/python/docs/index.php#on-subscribe

无需订阅 python Paho MQTT 库即可接收消息

Receiving messages without subscribing in python Paho MQTT library

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