Is there anyway to detect if a Tkinter variable has been changed by the user and not internally?

Question

I have a larger program I am working on that loads and displays objects that a user creates. Each object has specific settings that can be changed by check buttons in a menu. When an object is loaded the display settings are overwritten using .set() to whatever specific settings the object has. I am using .trace() to modify the display when the settings are changed. The problem is I want the callback to happen only when the user changes and not when a new object is loaded and the settings are overwritten.

To make the problem simpler say I have 4 checkboxes and a function that prints the index that the user clicked. Then I have a button that checks only the even boxes. Is there a way I could make it so my callback function printIndex only happens when the user clicks the checkbox and not when the button sets the values??

Here is the simplified code:

import tkinter as tk

def printIndex(index):
    print("User clicked {}".format(index))

def setOnlyOdds():
    print("\nSetting only Evens")
    for i in range(4):
        value = True if (i + 1 ) % 2 == 0 else False
        print("{} is being set to {}".format(i+1,value))
        bools[i].set(value)

root = tk.Tk()
root.title("Qhat Apps")
root.grid()

bools = [] # 4 total 
checkBoxes = [] # 4 total
for i in range(4):
    bools.append(tk.BooleanVar())
    checkBoxes.append(tk.Checkbutton(root,variable = bools[i]))
    checkBoxes[i].grid(column = 1, row = i)

label1 = tk.Label(text = "1").grid(column = 0, row = 0)
label2 = tk.Label(text = "2").grid(column = 0, row = 1)
label3 = tk.Label(text = "3").grid(column = 0, row = 2)
label4 = tk.Label(text = "4").grid(column = 0, row = 3)

bools[0].trace("w", lambda *args: printIndex(1))
bools[1].trace("w", lambda *args: printIndex(2))
bools[2].trace("w", lambda *args: printIndex(3))
bools[3].trace("w", lambda *args: printIndex(4))

evens = tk.Button(root,text = "Only Evens", command = setOnlyOdds).grid(columnspan = 2)

root.mainloop()

Thanks in advance!

Why not just set the output callback on the `Checkbutton` instances, since it's when those are clicked that you want output to occur? — Karl Knechtel, Apr 08 '20 at 16:45
@KarlKnechtel, I changed the lines bools[0].trace... to checkBoxes[0].config(command = printIndex(1)) and it still doesn't work??? Is another way I should go about it? — Jadon Erwin, Apr 08 '20 at 16:57
Read [Why is Button parameter “command” executed when declared?](https://stackoverflow.com/q/5767228/7432) — stovfl, Apr 08 '20 at 17:18

score 1 · Accepted Answer · answered Apr 08 '20 at 18:46

So I changed my checkbutton to have the command instead of putting it in trace.

import tkinter as tk

def printIndex(index):
    print("User clicked {}".format(index))

def setOnlyOdds():
    print("\nSetting only Evens")
    for i in range(4):
        value = True if (i + 1 ) % 2 == 0 else False
        print("{} is being set to {}".format(i+1,value))
        bools[i].set(value)

root = tk.Tk()
root.title("Qhat Apps")
root.grid()

bools = [] # 4 total 

for i in range(4):
    bools.append(tk.BooleanVar())
checkBox1 = tk.Checkbutton(root,variable = bools[0], command = lambda: printIndex(1))
checkBox2 = tk.Checkbutton(root,variable = bools[1], command = lambda: printIndex(2))
checkBox3 = tk.Checkbutton(root,variable = bools[2], command = lambda: printIndex(3))
checkBox4 = tk.Checkbutton(root,variable = bools[3], command = lambda: printIndex(4))
checkBox1.grid(column = 1, row = 0)
checkBox2.grid(column = 1, row = 1)
checkBox3.grid(column = 1, row = 2)
checkBox4.grid(column = 1, row = 3)

label1 = tk.Label(text = "1").grid(column = 0, row = 0)
label2 = tk.Label(text = "2").grid(column = 0, row = 1)
label3 = tk.Label(text = "3").grid(column = 0, row = 2)
label4 = tk.Label(text = "4").grid(column = 0, row = 3)

evens = tk.Button(root,text = "Only Evens", command = setOnlyOdds).grid(columnspan = 2)

root.mainloop()

Is there anyway to detect if a Tkinter variable has been changed by the user and not internally?

1 Answers1