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Python client - subclassing [message #1233523]

Sun, 19 January 2014 22:29

Roy Barkas

Messages: 1
Registered: January 2014

Junior Member

The example that ships with the Python client does this:

class MyMQTTClass:
    def __init__(self, clientid=None):
        self._mqttc = mqtt.Client(clientid)

Before I saw the example I had developed a module that subclasses the client like:

class MqClient(client.Client):
    def __init__(self, name):
        client.Client.__init__(self, name)

Is there any reason why the subclassing method shown in the repo example is preferable to the one I'm using? Mine seems to work fine at this point.
The major difference is that the repo example creates an internal _mqttc object where my method subclasses the client

The (admittedly minor) advantage of my approach is that I can do:

mqClient = MqClient({'name':'test123', etc etc})
mqClient.subscribe(mytopic, myqos)

as opposed to the repo example requiring:

mqClient = MyClient('test123')
mqClient._mqtcc.subscribe(mytopic, myqos)

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Re: Python client - subclassing [message #1234392 is a reply to message #1233523]

Tue, 21 January 2014 21:58

Ian Craggs

Messages: 76
Registered: March 2013

Member

Hi Roy,

from a purely OO point of view, both approaches have their own characteristics which might suit different circumstances. Roger's example is not a subclass at all really, it's a class which has the mqtt client as an attribute. This means that the methods on the mqtt client are not directly available on MyMQTTClass, whereas on your class MqClient they are.

It depends what you want from your class. Maybe Roger will explain his reasoning...

Ian

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Re: Python client - subclassing [message #1234555 is a reply to message #1234392]

Wed, 22 January 2014 08:54

Roger Light

Messages: 45
Registered: September 2013

Member

Hi Roy,

I think the confusion here is just some unfortunate naming. The sub-class.py name doesn't actually refer to subclassing, it's a duplicate of the sub[scribing-client].py example but using functions within a class instead of standalone. By that measure, the subclassing example would be sub-subclass.py - elegant! Smile

In general, I agree with Ian that it depends what you want with your class. It may make more sense to have the MQTT features hidden in your application, or it may not.

Cheers,

Roger

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Re: Python client - subclassing [message #1718416 is a reply to message #1233523]

Wed, 23 December 2015 13:47

Martin Frlin

Messages: 2
Registered: December 2015

Junior Member

Sorry for resurrecting an old thread.

Any advice on how to subclass the Client?

I'm having troubles with callbacks.

This does not work for example:

class MQTTClient(mqtt.Client):

    def __init__(self, client_id='', clean_session=True,  userdata=None, protocol=mqtt.MQTTv31):
        super(MQTTClient, self).__init__(client_id, clean_session,  userdata, protocol)
        self.on_message = self.on_message_cb

    def on_message_cb(self, userdata, message):
        print mesage.payload

The best thing would be if on_message would not be defined in __init__ but it would be an emptu funtion in mqtt.Client. Then you would just redefine the function in your own class.

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