Change variable with button in tkinter

Question

The code the problem is part of is fairly big so I'm drafting a fingerpainting version here.

import tkinter

variable = "data"

def changeVariable():
    variable = "different data"

def printVariable():
    print(variable)

window = tkinter.Tk
button1 = tkinter.Button(window, command=changeVariable)
button1.pack()
button2 = tkinter.Button(window, command=printVariable)
button2.pack()

So in this example, I press the first button to change 'variable', then the second button to print it. But "data" is printed instead of "different data". I searched around a bit and decided to use global before defining the variable in both the main code and in the functions, so the code looked like this.

import tkinter

global variable
variable = "data"

def changeVariable():
    global variable
    variable = "different data"

def printVariable():
    global variable
    print(variable)

window = tkinter.Tk()
button1 = tkinter.Button(window, command=changeVariable)
button1.pack()
button2 = tkinter.Button(window, command=printVariable)
button2.pack()

window.mainloop()

But now it says 'name 'variable' is not defined'.

Essentially, how can I get the variable 'variable' to change with a button in tkinter? Was I wrong to think of using global?

Answer 1

Your use of global is a bit off. You do not need to define global all over the place. Lets break it down a little.

You don't need to define global namespace in the global namespace.

from tkinter import *
window = Tk()
myvar = "data" # this variable is already in the global namespace

This tells the function to check global namespace when its interacts with the variable myvar.

def changeVariable():
    global myvar
    myvar = "different data"

This print statement works because it checks the global variable namespace after it has check the other namespaces without finding the variable myvar.

def printVariable():
    print(myvar)

button1 = Button(window, command = changeVariable)
button1.pack()
button2 = Button(window, command = printVariable)
button2.pack()

window.mainloop()

So if we put this code together we will get the desired result.

from tkinter import *
window = Tk()
variable = "data"

def changeVariable():
    global variable
    variable = "different data"

def printVariable():
    print(variable)

button1 = Button(window, command = changeVariable)
button1.pack()
button2 = Button(window, command = printVariable)
button2.pack()

window.mainloop()

This results in a window that looks like this:

and the result if we press the bottom button first then the top button then the bottom button again we get: