Detecting change in the value of python variables

Question

Is there some way to call a function each-time the value of a variable changes, in python?

Something like a listener, perhaps?

Specifically, I'm referring to the case where only the variable is shared between scripts like a GAE-Session. (Which uses Cookies, Memcache, etc to share data)

Example: ScriptA & ScriptB, share a session variable. When Script B changes it, SctiptA has to call a method to handle that change.

When you talk about 'Script A' and 'Script B', are you expecting communication between handlers for different requests? This isn't possible - one app may be running on multiple, distinct, servers. — Nick Johnson
– Nick Johnson, Commented Sep 26, 2011 at 3:33

Soner from The Ottoman Empire · Accepted Answer · 2018-07-17 15:29:34Z

6

Use properties. Only mutable values can change to begin with.

class Something(object):
    @property
    def some_value(self):
        return self._actual
    @some_value.setter
    def some_value(self, value):
        print ("some_value changed to", value)
        self._actual = value

edited Jul 17, 2018 at 15:29

Soner from The Ottoman Empire

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answered Sep 24, 2011 at 17:33

Jochen Ritzel

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1 Comment

Nick Johnson Over a year ago

I didn't know you could define writable properties like this in Python. I'm not sure if I like it or not, but at least it's novel.

RavuAlHemio · Accepted Answer · 2011-09-24 17:01:26Z

3

If the variable is a class attribute, you may define __setattr__ in the class.

answered Sep 24, 2011 at 17:01

RavuAlHemio

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2 Comments

S P Over a year ago

Looks very promising, but I don't control the sessions class. But if there were some way that the method could be declared dynamically (like JavaScript) or something, then it might work...

steveha Over a year ago

Can you subclass the 'sessions class' and pass around an instance of the subclass? The subclass would work exactly like the base class, but would have your extra features added in.

ogre · Accepted Answer · 2015-09-08 03:35:09Z

0

You Probably need extend type "int" or "float", and override the function "setattr", make it to response in your own way.

class MyInt(int):
    ...
    def __setattr__(self, attr, val):
        ...
        super(MyInt, self).__setattr__(attr, val)
        ...

var = MyInt(5)

answered Sep 8, 2015 at 3:35

ogre

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Comments

Jakob Bowyer · Accepted Answer · 2011-09-24 16:58:23Z

-1

For this case (And I can't believe im saying the words) sounds like situation for setters

def set_myvar(x):
    my_var = x
    some_function()
    return my_var

answered Sep 24, 2011 at 16:58

Jakob Bowyer

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4 Comments

S P Over a year ago

Nice, but in my case, sadly the function can't be shared as its GAE-sessions I'm talking about.

Jakob Bowyer Over a year ago

Then just call the function after the variable change

S P Over a year ago

I was hoping for another way... Especially 'cos both the scripts, sharing the variable don't necessarily share the function.

Jakob Bowyer Over a year ago

They would need to share the function for the variable to call the function when its updated

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