how to listene for variable change in python

userc2Ad1 0
Viewed 38

I want to run a function when a variable changes its value for example I have a variable called 'v1' the default value is ZERO
Let's say I have GUI app and I press button, this button only has one function that add one to 'v1' I want another function run when it detects v1 had been change like print('v1 change'), run when a button click or user input or anything that can change its value , JavaScript has onchange function dose python has something like that?

variables python-3.x python
1 Answers

Standard variables in Python don't have this functionality.

Some GUIs use special class(es) to keep value - and it may need to use functions to get and set value in class. And it has method to add functions which should be executed when variable change value. These added functions are called callbacks

Minimal working example

class Var:
    
    def __init__(self, value=0):
        
        self._value = value
        self._callbacks = []
        
    def set(self, value):
        old_value = self._value
        self._value = value

        # execute functions when it set value
        for func in self._callbacks:
            func(old_value, value)
        
    def get(self, value):
        return self._value
    
    def add_callback(self, func):
        if func not in self._callbacks:
            self._callbacks.append(func)

    def remove_callback(self, func):
        if func in self._callbacks:
            self._callbacks.remove(func)

def myfunc(old, new):
    print("changed:", old, new)
    
v = Var()

v.add_callback(myfunc)

v.set(10)   # changed: 0 10
v.set(-10)  # changed: 10 -10

v.remove_callback(myfunc)

v.set(0)    # without text
furas0
Related