Python tkinter StringVar() trace method

str1=tk.StringVar(master,value,name)
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() methods to manage data and trigger call back functions


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

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

Example of uses of StringVar

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() 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("www.plus2net.com")  # Adding a title

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

l3 = tk.Label(my_w,  textvariable=str1, width=15 )  
l3.grid(row=2,column=1)

b1 = tk.Button(my_w,text='Update',command=lambda:str1.set('Welcome'))  
b1.grid(row=2,column=3)

str1.trace('w',my_r)

my_w.mainloop() 
Output
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

Length of StringVar

We can use len
len(str1.get())

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(), by using this we can track the changes and trigger function. Here are some scripts using trace() 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() 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()
from tkinter import *
my_w.geometry("400x150")  # Size of the window width x height
my_w.title("plus2net.com")  # Adding a title
 
e1_str=tk.StringVar() # declaring a StringVar()
e1 = tk.Entry(my_w,textvariable=e1_str,bg='yellow',font=28) # Entry box
e1.grid(row=0,column=0,padx=10,pady=30) 

l1 = tk.Label(my_w,  text='No of Chars here' ,font=28)  # added one Label 
l1.grid(row=0,column=1)

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

e1_str.trace('w',my_upd) # triggers on change of StringVar
my_w.mainloop()
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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    22-03-2021

    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

    23-03-2021

    Thanks, Let us use str1

    Post your comments , suggestion , error , requirements etc here





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