升级paho-mqtt 2.x后on_disconnect回调参数数量异常问题

问题描述 #

将Python项目升级至paho-mqtt 2.x版本后，已按官方文档适配各类回调函数，但on_disconnect()回调始终提示参数数量错误：系统显示传入了5个参数（预期为4个），但代码中该回调仅定义了4个参数，无法定位多余参数来源。原代码在paho-mqtt 1.x版本可正常运行。

原问题代码 #

def connect_mqtt(client_id, username, password) -> mqtt_client:
    def on_connect(client, userdata, flags, reason_code, properties):
        if reason_code == 0:
            writeLog("Connected to MQTT Broker: " + client._client_id.decode('utf-8'),0)   
            if client._client_id.decode('utf-8') == client_id_p:
                globals()["flag_connected_p"] = True                    
            if client._client_id.decode('utf-8') == client_id_s:
                globals()["flag_connected_s"] = True
                writeLog("Subscribing to topic: " + topic_s + " With client: " + client._client_id.decode('utf-8'),0)
                subscribe(client, topic_s)
        else:
            writeLog("Failed to connect to client: " + client._client_id.decode('utf-8') + ", error code:" + str(reason_code),0)
            writeLog("1: Connection refused - incorrect protocol version",0)
            writeLog("2: Connection refused - invalid client identifier",0)
            writeLog("3: Connection refused - server unavailable",0)
            writeLog("4: Connection refused - bad username or password",0)
            writeLog("5: Connection refused - not authorised",0)            

    def on_disconnect(client, userdata, reason_code, properties):
        if reason_code == 0:
            writeLog("Requested disconnect MQTT excuted sucesfully for client: !" + client._client_id.decode('utf-8'),0)                   
        else:
            writeLog("Unexcpected disconnect from MQTT for client: " + client._client_id.decode('utf-8') + ", rc=" + str(reason_code),0)
        if client._client_id.decode('utf-8') == client_id_p:
            globals()["flag_connected_p"] = False
        if client._client_id.decode('utf-8') == client_id_s:
            globals()["flag_connected_s"] = False             

    def on_publish(client, userdata, mid, reason_codes, properties):
        writeLog("published message "+ str(mid) + " From client: " + client._client_id.decode('utf-8') + " sucesfully",1) 

    client = mqtt_client.Client(mqtt.CallbackAPIVersion.VERSION2,client_id, CleanSession)
    client.username_pw_set(username, password)
    client.on_connect = on_connect
    client.on_disconnect = on_disconnect
    client.on_publish = on_publish
    client.max_inflight_messages_set(1000)
    client.connect(broker, port)
    return client

问题原因及解决方案 #

在Paho-MQTT 2.x的CallbackAPIVersion.VERSION2回调规范中，on_disconnect的正确参数签名是5个参数：client, userdata, reason_code, properties, disconnect_flags。你当前的代码只定义了4个参数，缺少最后一个disconnect_flags参数，导致系统传参时不匹配。

修改后的on_disconnect函数如下（即使暂时不需要使用disconnect_flags，也需要保留参数占位）：

def on_disconnect(client, userdata, reason_code, properties, disconnect_flags):
    if reason_code == 0:
        writeLog("Requested disconnect MQTT excuted sucesfully for client: !" + client._client_id.decode('utf-8'),0)                   
    else:
        writeLog("Unexcpected disconnect from MQTT for client: " + client._client_id.decode('utf-8') + ", rc=" + str(reason_code),0)
    if client._client_id.decode('utf-8') == client_id_p:
        globals()["flag_connected_p"] = False
    if client._client_id.decode('utf-8') == client_id_s:
        globals()["flag_connected_s"] = False             

这个参数是2.x版本新增的，用于传递断开连接的标志位信息，升级时需要同步适配该回调的参数数量。

内容的提问来源于stack exchange，提问作者Pepijn De Winter