Python tkinter BooleanVar() trace


For an BooleanVar() so we can check the different modes of this variable, like this

r :Read - the variable is read by someone
w :Write- the variable is written by someone.
u :undefined – The variable is deleted

BooleanVar()
Here is an example which uses w ( write ) mode to display the value of the variable when ever it changes.
We used one Button and used on Click event to change the value of this variable bv1 from False to True.
b1 = tk.Button(my_w,text='Update',command=lambda:bv1.set(True))
This triggers the trace() which used the callback function my_r() to print the value of the variable ( True ) to our console.
import tkinter as tk
from tkinter import *
my_w = tk.Tk()
my_w.geometry("300x100")  # Size of the window 
my_w.title("www.plus2net.com")  # Adding a title

def my_r(*args):
    print(bv1.get()) #  Print when variable changes.
	
bv1 = tk.BooleanVar(my_w) # declare Boolean Variable 
bv1.set(0) 

b1 = tk.Button(my_w,text='Update',command=lambda:bv1.set(1))  
b1.grid(row=2,column=3,padx=30,pady=10)

bv1.trace('w',my_r) # monitor the change of variable
my_w.mainloop()
Output
True
Use StringVar() for handling String data
Use DoubleVar() for handling Float Data
Use IntVar() for handling Integer Data

set() and get() methods of BooleanVar

In above examples we used set() method to assign data to the Boolean variable and to read the data stored we used get() method. These two methods are frequently used in our scripts.
bv1.set(1) # assign value to bv1
print(bv1.get()) # display the value assigned to bv1

Initializing BooleanVar

We can using set() method to assign data to BooleanVar, after declaring or we can assign value while declaring the string variable.
bv1=tk.BooleanVar(value=True) # Assign value to bv1

Length of BooleanVar

Before using len function, we have to convert the BooleanVar to string by using str()
print(len(str(bv1.get()))) # 4 

Normal Variable and BooleanVar

BooleanVar() is a class in Tkinter. In a GUI application we require various events to be captured to trigger different functions (requirements). If we use normal Python variable with widgets then monitoring the changes is difficult. However by using a BooleanVar we can monitor the changes and trigger functions based on the requirements.

StringVar() DoubleVar() IntVar()

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