[Tkinter] How to terminate a loop in python with button in GUI

[Joni_Engr]

Hi, in my GUI I have a function with a while loop inside which is ruining when START button is pressed. The function drive a microcontroller continuously. I am wondering is it possible to terminate the running function when the STOP button is pressed ?

[Yoriz]

A while loop will block a GUI's mainloop, the while loop won't block if it running in a separate thread.
A flag variable that can be set outside of the while loop, can be checked in the while loop to decide if the while loop should continue looping or stop.

[Joni_Engr]

Yes exactly, this I want to implement.

A flag variable can be set outside of the while loop when the STOP button is pressed while the while loop is running in a separate thread. In side the while loop the flag variable decide the while loop should continue looping or stop.

Question how to run a while loop in a separate thread when START button in GI is pressed and how to assign the main GUI in another thread ?

[Yoriz]

You don't mention which GUI framework you are using, the following links give an idea of a way to use threads with tkinter and wxpython:
[Tkinter] How to deal with code that blocks the mainloop, freezing the gui
[WxPython] How to deal with code that blocks the mainloop, freezing the gui

[Joni_Engr]

I am using tkinter. Here is my code. There are two buttons on the GUI. STRAT and STOP. If I press START then a counter up to 30 should be printed on the shell but if I press STOP anytime before the counter reach 30 then the loop should terminate and back to the man loop of GUI. Where should I add thread ?

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import time   from tkinter import * root = Tk() root.title("Loop Terminate")   time.sleep(0.5)   # Function button_stop def button_stop(): # If the STOP button is pressed then terminate the loop   i = 1   # Function button_start def button_start():   j = 1   while j <= int(30):     print("Loop Index = " + str(j))     time.sleep(0.5)     j = j+1   # Button START button_start =  Button(root, text = "START", padx=53, pady=20, command=button_start) button_start.grid(columnspan=1, row=1,column=0)   # Button STOP button_stop =  Button(root, text = "STOP", padx=44, pady=20, command=button_stop) button_stop.grid(row=2,column=0)

[Yoriz]

Please see Namespace flooding with * imports
Here is an example of what you want to do

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import threading import time import tkinter as tk from dataclasses import dataclass, field   btn_state = {True: "normal", False: "disabled"}     @dataclass class Task:     sleep_duration: float = field(default=0.5)     _run_loop: bool = field(default=False, init=False)       def start(self) -> None:         self._run_loop = True         for number in range(1, 31):             if not self._run_loop:                 return             print(f"Loop index = {number}")             time.sleep(self.sleep_duration)         self._run_loop = False       def stop(self) -> None:         self._run_loop = False       def threading_start(self) -> None:         thread = threading.Thread(target=self.start)         thread.start()       @property     def running(self) -> bool:         return self._run_loop     class TaskFrame(tk.Frame):     def __init__(self, *args, **kwargs):         super().__init__(*args, *kwargs)         self.task = Task()         self._create_contrls()         self._create_layout()         self._create_binds()         self._update_btn_states()       def _create_contrls(self) -> None:         self.btn_start = tk.Button(master=self, text="START", padx=53, pady=20)         self.btn_stop = tk.Button(master=self, text="STOP", padx=44, pady=20)       def _create_layout(self) -> None:         self.btn_start.grid(columnspan=1, row=1, column=0, padx=2, pady=2)         self.btn_stop.grid(row=2, column=0, padx=2, pady=2)       def _create_binds(self) -> None:         self.btn_start.bind("<Button-1>", self._on_btn_start)         self.btn_stop.bind("<Button-1>", self._on_btn_stop)       def _on_btn_start(self, event: tk.Event) -> None:         if self.task.running:             return         self.task.threading_start()       def _on_btn_stop(self, event: tk.Event) -> None:         if not self.task.running:             return         self.task.stop()       def _update_btn_states(self) -> None:         self.btn_start.configure(state=btn_state[not self.task.running])         self.btn_stop.configure(state=btn_state[self.task.running])         self.after(500, self._update_btn_states)     def main():     app = tk.Tk()     app.title("Loop Terminate")     main_frame = TaskFrame(app)     main_frame.pack()     app.mainloop()     if __name__ == "__main__":     main()

[deanhystad]

Your program doesn't work because tkinter processes things like button presses while your program is not running. If you enter a loop, tkinter stops running and waits for the loop to complete. To know that a button was pressed you cannot be waiting for it to happen.

You can get around this by having a thread for the GUI and another for the background task, but is it required? Why are you waiting? Could you implement the logic in some other way. I would implement your example using tkinter.after().

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import tkinter as tk   class Counter():     '''Periodically print counter value to stdout'''     def __init__(self, parent, end, start=0, increment=1, interval=1000):         self.parent = parent         self.start_value = start         self.end_value = end         self.increment = -increment if (end - start) * increment < 0 else increment         self.interval = interval         self.value = start         self.running = False       def start(self):         '''Start counter'''         self.value = self.start_value         self.running = True         self.doit()       def stop(self):         '''Stop counter'''         self.running = False       def doit(self):         '''Called periodically to incrementer counter and print value'''         if self.running:             print('Counter value =', self.value)             self.value += self.increment             self.running = (self.end_value - self.value) * self.increment > 0             if self.running:                 self.parent.after(self.interval, self.doit)   root = tk.Tk() root.title("Loop Terminate") counter = Counter(root, 0, 10)   # Function button_stop def button_stop():     counter.stop()   # Function button_start def button_start():     counter.start()   # Button START button_start =  tk.Button(root, text = "START", padx=53, pady=20, command=button_start) button_start.grid(columnspan=1, row=1,column=0)   # Button STOP button_stop =  tk.Button(root, text = "STOP", padx=44, pady=20, command=button_stop) button_stop.grid(row=2,column=0)   root.mainloop()