How to do stuff when a variable's value is modified?

Question

How to do stuff when a variable's value is modified?

try:                        # In order to be able to import tkinter for
    import tkinter as tk    # either in python 2 or in python 3
except ImportError:
    import Tkinter as tk


def do_stuff():
    demo['text'] = var


if __name__ == '__main__':
    root = tk.Tk()
    var = "Value"
    demo = tk.Label(root, text=var)
    demo.pack()
    var = "New Value" # call do_stuff()
    tk.mainloop()

For example, I want to call do_stuff whenever var is modified.

Answer 1

One way would be to make use of Variable classes(BooleanVar, DoubleVar, IntVar, StringVar, Variable), and their trace_add method. Which allows to call a method when the value the instance holds is modified.

try:                        # In order to be able to import tkinter for
    import tkinter as tk    # either in python 2 or in python 3
except ImportError:
    import Tkinter as tk


def do_stuff(*_):   # trace_add implicitly passes 3 internal arguments
    demo['text'] = var.get() # to obtain the value var holds


if __name__ == '__main__':
    root = tk.Tk()
    var = tk.StringVar(value="Value")
    #var = tk.BooleanVar(value=True)
    #var = tk.DoubleVar(value=2.78)
    #var = tk.IntVar(value=0)
    #var = tk.Variable(value=["Value", True, 2.78, 0])
    demo = tk.Label(root, text=var.get())
    demo.pack()
    var.trace_add('write', do_stuff)    # call do_stuff when var is set
    var.set("New Value")    # to replace the value var holds
    #var.set(False)
    #var.set(3.14)
    #var.set(42)
    #var.set(["New Value", False, 3.14, 42])
    tk.mainloop()

---

Also further note that, while not always practical, some widgets have variable options(variable, textvariable, listvariable) that allows the value they hold to be in sync with that of a Variable class object. Below example makes use of that:

try:                        # In order to be able to import tkinter for
    import tkinter as tk    # either in python 2 or in python 3
except ImportError:
    import Tkinter as tk


if __name__ == '__main__':
    root = tk.Tk()
    var = tk.StringVar(value="Value")
    demo = tk.Label(root, textvariable=var)
    demo.pack()
    var.set("New Value")    # label is in sync right-away
    tk.mainloop()

---

Another way would be to make use of attribute assignment magic method(__setattr__) of an object, when working with classes.

For example the demo label will be updated each time self.string attribute is modified:

try:                        # In order to be able to import tkinter for
    import tkinter as tk    # either in python 2 or in python 3
except ImportError:
    import Tkinter as tk


class MyLabel(tk.Label):
    def __init__(self, master, var="", *args, **kwargs):
        tk.Label.__init__(self, master, *args, **kwargs)
        self.var = var


    def __setattr__(self, name, value):
        self.__dict__[name] = value
        if name == 'var':
            self.do_stuff()


    def do_stuff(self):
        self['text'] = self.var


if __name__ == '__main__':
    root = tk.Tk()
    demo = MyLabel(root, "Value")
    demo.pack()
    demo.var = "New Value"
    tk.mainloop()