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?

Your code works for me. Are you sure you didn't misspell "variable" in there somewhere? — Novel
– Novel, Commented May 31, 2017 at 19:43
Your second example works for me once I make two changes: window = tkinter.Tk() and add window.mainloop() at the end. — FamousJameous
– FamousJameous, Commented May 31, 2017 at 19:54
What @quamrana said: the global directive is used inside a function to tell the function to look up the name in the global namespace. It doesn't make sense to use it in the global namespace itself. But really, it's a good idea to avoid using modifiable globals because they break program modularity. The neat way to do this is to define your GUI as a class, and then you can use instance attributes to save state data. BTW, it's a common practice to do import tkinter as tk. Then you can do stuff like button = tk.Button(window). — PM 2Ring
– PM 2Ring, Commented May 31, 2017 at 20:03

Mike - SMT · Accepted Answer · 2017-06-02 14:50:01Z

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:

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