how to trigger function in another object when variable changed. Python

Question

As far as I know, this is like an Observer pattern. Scenario: A Center object keeps a list (queue) of all its clients. I'm using Twisted.

One of client objects changes a variable in center object OR notify the center to change the variable,
and then the center object detects the change immediately;
then as soon as the detection, the center object invoke some function of next object in queue
After the client changed the variable, the client object will be eliminated. The center will take care of next client object. So I imagine there's no any function chain between these objects. So it's a little bit different from observer pattern. (How to address this issue? Correct me if I'm wrong.)

following code is just for demo only:

    class client():
        def change(self):
            self.center.va = 1

        def inqueue(self):
            self.center.queue.enqueue(self)

        def function(self):
            pass

    class center():
        def __init__(self):
            self.queue = None
            self.va = 0

        ####  When the self.va changes, this func will be invoked
        def whenChanged(self):
            next = self.queue.dequeue()
            next.function()

Perhaps define a setter/getter property.

Stephen B
– Stephen B

2016-09-27 16:43:16 +00:00
Commented Sep 27, 2016 at 16:43 — Stephen B
– Stephen B, Commented Sep 27, 2016 at 16:43

chepner · Accepted Answer · 2016-09-27 16:43:07Z

10

Make va a property.

class Center():
    def __init__(self):
        self.queue = None
        self._va = 0

    @property
    def va(self):
        return self._va

    @va.setter
    def va(self, value):
        self._va = value
        self.whenChanged()

    def whenChanged(self):
        next = self.queue.dequeue()
        next.function()

answered Sep 27, 2016 at 16:43

chepner

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12 Comments

Henry Over a year ago

I assume that va setter will called in first client, right? So does this solve the issue that the first client will be eliminated before this setter invocation return?

chepner Over a year ago

The setter is invoked immediately upon self.center.va = 1, before client.change exits, so it runs before the client goes away.

Henry Over a year ago

What if the client must leave while or before this invocation chain returns?

Henry Over a year ago

btw, i cannot sue property since I'm inheriting old style class.

chepner Over a year ago

You appear to have left a lot of important context out of your question.

|

Moinuddin Quadri · Accepted Answer · 2021-03-01 20:50:19Z

6

Whenever a property of class is changed, setattr() function is called. You can override this by defining __setattr__(self, property, value) function in your class.

You need to make your required function call within this __ setattr__(). Below is the sample example based on your requirement:

class Centre(object):
    def __init__(self):
        self.queue = None
        self.va = 0

    def whenChanged(self):
        next = self.queue.dequeue()
        next.function()

    def __setattr__(self, key, value):
        self.key = value
        self.whenChanged()  # <-- Your function

Whenever you will attempt to change the value of any of class's property, this __settattr__ function will be called.

edited Mar 1, 2021 at 20:50

answered Sep 27, 2016 at 16:46

Moinuddin Quadri

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6 Comments

Henry Over a year ago

I'm using Twisted, and it seems that I cannot use new-style class. Can I still use this?

Moinuddin Quadri Over a year ago

It is the part of core Python. You can use it with any flavor of Python. Made an edit with sample code

Henry Over a year ago

you said: " this __settattr__ function will be called." but __setattr__ will be called by who?

Moinuddin Quadri Over a year ago

Did you checked the document linked to __setattr__()? Whenever a value is assigned to class's property,this function is called. For example, on doing self.va = 9, for assigning the value as 9, this function is called

Henry Over a year ago

Yes, I checked that out. So I assume that if I have this assignment: center.va = 9 in a client object, then the center object will invoke the __setattr__() and there's no function invocation chain between center object and client object, right?

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