Paho MQTT Python Client: No exceptions thrown, just stops

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I try to setup a mqtt client in python3. This is not the first time im doing this, however i came across a rather odd behaviour. When trying to call a function, which contains a bug, from one of the callback functions (on_connect or on_message), python does not throw an exception (at least it is not printed), it just stops there. I tied together a short example, that reproduces that behaviour.

Does someone has an idea?

import paho.mqtt.client as mqtt

import re
import os.path

import json
from termcolor import colored

client = mqtt.Client()

def func():
    test = 1 + "1"
    print("Should never reach that")

def on_connect(client, userdata, flags, rc):
    """Establishes connection to broker
    """
    print("Connected to broker with result code " + str(rc))
    client.subscribe("test")

def on_message(client,userdata,msg):
    print("Recieved message on " + msg.topic)
    params = {}
    if msg.topic == "test":

        print("Invoke func")
        func()

if __name__ == "__main__":
    client.on_connect = on_connect
    client.on_message = on_message

    client.connect("localhost",1883,60)

    client.loop_forever()

This is the output when sending a message to the topic "test":

Connected to broker with result code 0
Recieved message on test
Invoke func

When calling func() from main, i get the correct TypeError thrown. So something catches this exception in paho. I had a look at an olderproject (python2 though) and tried to recreate the behaviour. There the exception gets thrown correctly. What do i miss?

EDIT I can catch the exception by putting the func() call in a try block. How ever, it does not stop the execution of the program when not catched. I dont get why

This will be due to the fact that the on_message function is called by the network thread and it will be wrapping that call in a try block to stop errors in on_message from stopping that thread.

If you want to an error to stop the app then you should use your own try block in on_message and behave appropriately.

Exceptions and undefined variables are not raised within callback , This will be due to the fact that the on_message function is called by the network thread and it will be wrapping that call in a try block to stop errors in  I’m using paho to send and receive mqtt messages. So far it has been no problem to send the messages, I’m receiving them by using mosquitto. Now I want to read the messages by using a java client and I noticed that there has been less documentation about receiving the messages.

For anybody who comes across this and wonders why all exceptions inside of a mqtt callback are not thrown or at least not visible: In contrast to the python2 version of paho, the clients already catches ALL exceptions that occur when calling on of the user set callback functions. The output of this catch is then outputted to the on_log callback function. If this is not implemented by the user, there will be no visible output. So just add

def on_log(client, userdata, level, buff):

to your code, in which you can printout the exception descri

paho.mqtt.python/client.py at master · eclipse/paho.mqtt , Exceptions (e.g. missing variables) are not raised from within callback Install latest version of paho.mqtt python3 -mvenv v3 source v3/bin/activate pip install pip --upgr #!/usr/bin/env python -B import paho.mqtt.client as paho def however the best I can do is to stop the loop, with the script still hung. I'm getting a generic "Exception of type 'uPLibrary.Networking.M2Mqtt.Exceptions.MqttCommunicationException' was thrown." while trying to connect to an MQTT broker. I can connect to the broker just fine using MQTT.fx, and I can connect to another MQTT broker just fine using the M2MQTT libraries, so I'm thinking that the client isn't compatible

You can catch the errors using try + expect and then manually print the error message and pointer to the source of error using the traceback. This will give you mode details than using the on_log function.

import traceback

def on_message(client, userdata, msg):
    try:
        do_something(msg)
    except:
        traceback.print_exc()
        quit(0)

paho-mqtt · PyPI, Contribute to eclipse/paho.mqtt.python development by creating an account on GitHub. Only used when SSL context does not have check_hostname attribute. self._tls_insecure stop the network thread previously created with loop_start(). This call access message.topic in the callback, an exception will be raised. try:. Eclipse Paho Java MQTT client library. Paho is an Eclipse IoT project. - eclipse/paho.mqtt.java Join GitHub today. // no exception thrown if the filter was

Eclipse Paho, git clone https://github.com/eclipse/paho.mqtt.python This means that when client is restarted (not just reconnected, the object is When the queue is full, any further outgoing messages would be dropped. Loop Start/Stop Example. Hi Gents Thanks in advance for any suggestions! The problem is a Sonoff switch. Works fine using MQTT. OH2 appears to be bound to MQTT. Then it gets messy! : ( mqtt.cfg looks like this: mqtt-loc.url=tcp://192.…

MQTT client examples, The latest stable version is available in the Python Package Index (PyPi) and import paho.mqtt.client as mqtt # The callback for when the client receives a The reinitialise() function resets the client to its starting state as if it had just been created. When the queue is full, any further outgoing messages would be dropped. 7 Paho MQTT Python Client: No exceptions thrown, just stops 5 Paho MQTT Python Client: No exceptions thrown, just stops View more network posts →

Callbacks and synchronization in MQTT client applications, It also contains examples on how to use the MQTT client with Cumulocity IoT employing messages (called “static templates”) using C, Java, JavaScript, Python and C#. Besides the name and the type, the device does not have more information, So far we have only used MQTT to send data from the client to the server. There are client implementations in Lua, Python, C, JavaScript, Perl, Ruby, Java, and many others (Mens thinks COBOL doesn't have one, but that's a rare exception). He gave some small (but functional) Python examples, using the Paho MQTT library. The publishing code was one substantive line long, while the subscription example was made eight

Comments
  • Just dived into the paho module code and saw the same. This changed between the python2 and python3 version of paho. Thanks for your answer anyway, your explanation makes a lot of sense. Just have to make sure, to get the log of mqtt printed somehow, to actually see what failed
  • @ralvarez ask a new question and reference this one, tag it with python so the language experts can help you.