Python tkinter StringVar() trace_add() method

StringVar() in Tkinter is a variable class.
StringVar() is used to track changes to a string value within a Tkinter application.
master: (Optional)The variable is associated with, default value is parent window.
value:(Optional) We can set the initial value for the variable.
name : (Optional) Name given default is PY_VAR1

Tkitner StringVar() get(), set(), trace_add() methods to manage data and trigger call back functions


trace_add(self, mode, callback)
For an StringVar() we can check the different modes of this variable and trigger call back functions. This is the main advantage of using such variables.

read :Read - the variable is read by someone
write :Write- the variable is written ( updated ) by someone ( frequently used).
unset :undefined – The variable is deleted

or a list or tuple of such strings.
db1.trace_add(['write','read'],my_r) # callback when data changes

Example of uses of StringVar

We used one Button and on Click event of this we change the value of the String variable str1 from Hello to Welcome.
b1 = tk.Button(my_w,text='Update',command=lambda:str1.set('Welcome') )
This triggers the trace_add() which used the callback function my_r() to print the value of the variable ( Welcome ) to our console.
import tkinter as tk
from tkinter import *
my_w = tk.Tk()
my_w.geometry("300x200")  # Size of the window 
my_w.title("")  # Adding a title

def my_r(*args):
    print(str1.get()) #  Print when variable changes.
str1 = tk.StringVar(my_w) # declare StringVar()

l3 = tk.Label(my_w,  textvariable=str1, width=15 )  

b1 = tk.Button(my_w,text='Update',command=lambda:str1.set('Welcome'))  



set() and get() methods of StringVar

In above examples we used set() method to assign data to the string variable and to read the data stored we used get() method. These two methods are frequently used in our scripts.
str1.set('Hello') # assign value to str1
print(str1.get()) # display the value assigned to str1

Initializing StringVar

We can using set() method to assign data to StringVar after declaring or we can assign value while declaring the string variable.
str1=tk.StringVar(value='Option 1') # Assign value to str1

Resetting StringVar


Length of StringVar

We can use len

Normal Variable and StringVar

StringVar() 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 StringVar we can monitor the changes and trigger functions based on the requirements.

Example : Declare the password is strong or weak based on the number of chars entered by user. Here we can use StringVar and each time the variable changes its stored data, we can trigger the function checking the length of the input and show the message.

More Examples on StringVar()

The most frequently used method is trace_add(), by using this we can track the changes and trigger function. Here are some scripts using trace_add() method of StringVar().

Counting the number of chars

StringVar trace method to count number of chars entered As we enter data inside the entry box, the total number of chars entered will be displayed by the side ( Label l1 ) . Here StringVar() e1_str is connected to Entry widget e1 using textvariable option.

The trace_add() method of e1_str is used to trigger the function my_upd() whenever the e1_str data changes.

Inside the function my_upd() we read the user input by using e1.get() , then found out the number of chars by using len() and the converted the integer to string by using str() function.
import tkinter as tk
my_w = tk.Tk()
my_w.geometry("400x150")  # Size of the window width x height
my_w.title("")  # Adding a title
e1_str=tk.StringVar() # declaring a StringVar()
e1 = tk.Entry(my_w,textvariable=e1_str,bg='yellow',font=28) # Entry box

l1 = tk.Label(my_w,  text='No of Chars here' ,font=28)  # added one Label 

def my_upd(*args):
    l1.config(text=str(len(e1_str.get()))) # read & assign text to StringVar()

e1_str.trace_add('write',my_upd) # triggers on change of StringVar

Common StringVar() for Entry, Label and Button

Common StringVar in Label, button and Entry widget
Here we are using one common StringVar() str1 and we are updating text on a Label and Button by using one Entry box. Similarly on Click of a Button the text on Label and inside Entry widgets is also getting updated.
import tkinter as tk
my_w = tk.Tk()
my_w.geometry("450x200")  # Size of the window 
my_w.title("")  # Adding a title

def my_r(*args):
    print(str1.get()) #  Print when variable changes.
    bt1.config(text=str1.get()) # update button text

str1 = tk.StringVar(my_w) # declare StringVar()

l3 = tk.Label(my_w,  textvariable=str1, width=15 ,font=22,bg='yellow')  

bt1 = tk.Button(my_w,text='Update',command=lambda:str1.set('Welcome'))  

e1 = tk.Entry(my_w,textvariable=str1,bg='lightgreen',font=22)  

str1.trace_add('write',my_r) # callback function on write mode

my_w.mainloop() # keep the window open 

Updating Random text on a Label

We will take random element from a list and display the same on a Label with a time delay.
import tkinter as tk
my_w = tk.Tk()
import random
my_w.geometry("300x200")  # Size of the window 
my_w.title("")  # Adding a title

str1=tk.StringVar(value='Randome List') # Using StringVar



def my_fun():    
    random_element = random.choice(my_list) # random selection 
    str1.set(random_element) # Update StringVar 
    my_w.after(1000, my_fun) # call the function after a delay 
my_w.mainloop()  # Keep the window open

Triggering all Modes of StringVar() : an Example

Here we have three buttons, each button triggers one mode ( read, write or unset ) of event associated with StringVar(). Message is printed based on the callback function used.
import tkinter as tk
my_w = tk.Tk()
my_w.geometry("300x100")  # Size of the window 
my_w.title("")  # Adding a title

def my_fun():
    str2 = tk.StringVar(my_w,value='Welcome2') # 
    str2.trace_add('unset',my_unset) # variable is removed 

str1 = tk.StringVar(my_w,value='Welcome1') #
def my_write(*args):
    print(str1.get()) #  Print when variable changes.
    print('write is triggerd')
def my_read(*args):
    print(str1.get()) #  Print when variable accessed.
    print('read is triggerd')
def my_unset(*args):
    print('unset is triggerd')

bt1 = tk.Button(my_w,text='write',command=lambda:str1.set('plus2net'))  

bt2 = tk.Button(my_w,text='read',command=lambda:str1.get())  

bt3 = tk.Button(my_w,text='unset',command=my_fun)  

str1.trace_add('write',my_write) # callback when data changes
str1.trace_add('read',my_read) # callback when data read



Change Background colour based on number of chars entered
Interlinked Combobbox: how the changes in selection is triggered

DoubleVar() IntVar() BooleanVar()
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    Very bad to call a variable in Python "str"
    str = tk.StringVar(my_w) # declare StringVar()
    because str is build-in method to stringify data!
    class str(object='') --> built-in function in Python


    Thanks, Let us use str1

    Post your comments , suggestion , error , requirements etc here

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