How to stop a loop with a stop button in Tkinter?

Consider a case of running a process in a loop and we want to stop the loop whenever a button is clicked. Generally, in programming languages, to stop a continuous while loop, we use a break statement. However, in Tkinter, in place of the while loop, we use after() to run the defined function in a loop. To break the continuous loop, use a global Boolean variable which can be updated to change the running state of the loop.

For the given example,

• Create a global variable that works similar to the flag in a loop.

• Define two buttons, Start and Stop, to start and stop the execution.

• Define two functions, on_start() and on_stop(), to deliver the update on the loop.

Example

# Import the required libraries
from tkinter import *
from tkinter import ttk

# Create an instance of tkinter frame or window
win = Tk()

# Set the size of the window
win.geometry("700x350")

running = True

# Define a function to print the text in a loop
def print_text():
   if running:
      print("Hello World")

   win.after(1000, print_text)

# Define a function to start the loop
def on_start():
   global running
   running = True

# Define a function to stop the loop
def on_stop():
   global running
   running = False

canvas = Canvas(win, bg="skyblue3", width=600, height=60)
canvas.create_text(150, 10, text="Click the Start/Stop to execute the Code", font=('', 13))
canvas.pack()

# Add a Button to start/stop the loop
start = ttk.Button(win, text="Start", command=on_start)
start.pack(padx=10)

stop = ttk.Button(win, text="Stop", command=on_stop)
stop.pack(padx=10)

# Run a function to print text in window
win.after(1000, print_text)

win.mainloop()

Output

Run the above code to test the loop for a certain condition.

If we will run the above code and click the Start button, then it will print "Hello World" text in a loop that can be stopped by clicking the "Stop" Button.

Hello World
Hello World
Hello World
Hello World
Hello World

Process finished with exit code 0