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

Question

I have this program which beeps every second until it's stopped. The problem is that after I press "Start" and the beeps starts, I cannot click the "Stop" button because the window freezes. Any help is welcome.

#!/usr/bin/python
import Tkinter, tkMessageBox, time, winsound, msvcrt

running = True

Freq = 2500
Dur = 150

top = Tkinter.Tk()
top.title('MapAwareness')
top.geometry('200x100') # Size 200, 200

def start():
    sec = 0
    while running:
        if sec % 1 == 0:
            winsound.Beep(Freq, Dur)

        time.sleep(1)
        sec += 1

def stop():
    running = False

startButton = Tkinter.Button(top, height=2, width=20, text ="Start", command = start)
stopButton = Tkinter.Button(top, height=2, width=20, text ="Stop", command = stop)

startButton.pack()
stopButton.pack()

top.mainloop()

Answer 1

There are several things wrong with your code. First of all you shouldn't use time.sleep() in a Tkinter program because it interferes with the mainloop(). Instead one typically uses the universal widget method .after() to schedule a function to run after a specified delay.

Secondly you're not using global variables correctly. When you assign a value to a named variable in a function, it will create a local variable unless that name has been previous declared global. So for instance, your stop() function is creating a local variable named running and setting its value to 0, not changing the value of the global variable with the same name.

The previous rule doesn't apply to just referencing (reading) the current value of a variable. That is why it was OK to not have declared Freq and Dur globals in start().

Another problem is with the sec % 1 == 0 in your start() function. Any value % 1 is 0. To check odd/evenness use sec % 2.

Here's a working version which has also been reformatted to follow PEP 8 - Style Guide for Python Code more closely.

try:
    import tkinter as tk
except ModuleNotFoundError:
    import Tkinter as tk  # Python 2.
import winsound

FREQ = 2500
DUR = 150

after_id = None
secs = 0

def beeper():
    global after_id
    global secs

    secs += 1
    if secs % 2 == 0:  # Every other second.
        winsound.Beep(FREQ, DUR)
    after_id = top.after(1000, beeper)  # Check again in 1 second.

def start():
    global secs

    secs = 0
    beeper()  # Start repeated checking.

def stop():
    global after_id

    if after_id:
        top.after_cancel(after_id)
        after_id = None


if __name__ == '__main__':

    top = tk.Tk()
    top.title('MapAwareness')
    top.geometry('200x100')

    startButton = tk.Button(top, height=2, width=20, text="Start", command=start)
    stopButton = tk.Button(top, height=2, width=20, text="Stop", command=stop)
    startButton.pack()
    stopButton.pack()

    top.mainloop()

Update

Since this answer has become fairly popular, I'd like touch on another slightly more advanced topic — namely how making the code more object-oriented would simplify things by eliminating the need almost all of the global variables.

try:
    import tkinter as tk
except ModuleNotFoundError:
    import Tkinter as tk  # Python 2.
import winsound

FREQ = 2500
DUR = 150

class Application(tk.Frame, object):
    def __init__(self, master=None):
        super(Application, self).__init__(master)  # Call base class initializer.
        self.after_id = None
        self.secs = 0

        # Create widgets,
        startButton = tk.Button(top, height=2, width=20, text="Start", command=self.start)
        stopButton = tk.Button(top, height=2, width=20, text="Stop", command=self.stop)
        startButton.pack()
        stopButton.pack()

    def beeper(self):
        self.secs += 1
        if self.secs % 2 == 0:  # Every other second.
            winsound.Beep(FREQ, DUR)
        self.after_id = top.after(1000, self.beeper)  # Check again in 1 second.

    def start(self):
        self.secs = 0
        self.beeper()  # Start repeated checking.

    def stop(self):
        if self.after_id:
            top.after_cancel(self.after_id)
            self.after_id = None


if __name__ == '__main__':

    top = tk.Tk()
    app = Application()
    app.master.title('MapAwareness')
    app.master.geometry('200x100')
    app.mainloop()

Answer 2

from threading import Thread

def stttart():
    t1=Thread(target=start)
    t1.start()
def start():
    ...

def stop():
    ...

startButton = Tkinter.Button(top, height=2, width=20, text ="Start", command = stttart)
stopButton = Tkinter.Button(top, height=2, width=20, text ="Stop", command = stop)

startButton.pack()
stopButton.pack()

https://www.geeksforgeeks.org/how-to-use-thread-in-tkinter-python/

last year, this had been my big problem for some months

Answer 3

I used thread and global variable to fit your need. Not so complicated if you understand how they work. Just an addition of few lines and minor change to your existing line, and it works. Look through to see the changes made to your original code.

    #!/usr/bin/python
    import tkinter
    from tkinter import messagebox
    import time, winsound, msvcrt
    from threading import Thread

    running = True

    Freq = 2500
    Dur = 150

    top = tkinter.Tk()
    top.title('MapAwareness')
    top.geometry('200x100') # Size 200, 200

    def button_click():
        global running  #create global
        running = True

        # Create new thread
        t = Thread(target = start)
        # Start new thread
        t.start()

    def start():
        sec = 0
        while running:
            if running == False:
                break
            if sec % 1 == 0:
                winsound.Beep(Freq, Dur)

            time.sleep(1)
            sec += 1

    def stop():
        global running  #create global
        running = False

    startButton = tkinter.Button(top, height=2, width=20, text ="Start", command = button_click) #Change to call button_click instead start
    stopButton = tkinter.Button(top, height=2, width=20, text ="Stop", command = stop)

    startButton.pack()
    stopButton.pack()

    top.mainloop()

Answer 4

Beaten to the punch again but here goes nothing. As above use the after function to prevent the mainloop blocking.
See: tkinter: how to use after method

#!/usr/bin/python
import Tkinter, tkMessageBox, time

Freq = 2500
Dur = 150

top = Tkinter.Tk()
top.title('MapAwareness')
top.geometry('200x100') # Size 200, 200

def start():
    print ("Beep")
    top.after(1000, start)

def stop():
    print ("Stop")
    top.quit()

startButton = Tkinter.Button(top, height=2, width=20, text ="Start", command = start)
stopButton = Tkinter.Button(top, height=2, width=20, text ="Stop", command = stop)

startButton.pack()
stopButton.pack()
top.mainloop()

Answer 5

You code have top.mainloop() which has a while loop running inside it and on top of that you also have a while loop inside def start():. So it is like loop inside loop.

You can create a function that does what you want for the body of the loop. It should do exactly one iteration of the loop. Once it is done, it needs to arrange for itself to be called again some time in the future using after. How far in the future defines how fast your loop runs.

And you can then use after_cancel to cancel the event. Below code worked for me

import Tkinter, tkMessageBox, time, winsound, msvcrt

Freq = 2500
Dur = 150

top = tkinter.Tk()
top.title('MapAwareness')
top.geometry('200x100') # Size 200, 200

def start():
    global job1
    if running == True:
        winsound.Beep(Freq, Dur)
        job1 = top.after(1000, start)  # reschedule event in 1 seconds

def stop():
    global job1
    top.after_cancel(job1)

startButton = tkinter.Button(top, height=2, width=20, text ="Start", command = start)
stopButton = tkinter.Button(top, height=2, width=20, text ="Stop", command = stop)

startButton.pack()
stopButton.pack()
#top.after(1000, start)
top.mainloop()

Answer 6

The problem is that the while loop in start() blocks the GUI handler mainloop(). Try using Tk.after() in start():

def start(force=True):
    global running
    if force:
        running = True
    if running:
        winsound.Beep(Freq, Dur)
        top.after(1000, start, False)

And change stop():

def stop():
    global running
    running = False