How to break out of a while True: loop with a button

Here's an entire working code that you can save and run, I have used a thread for ease of use and responsiveness as well as system efficiency. There are really only 4 extra lines of code to produce the threaded alternative, I also added a few extra bits here and there. Making global variables writeable within functions for simplicity. Any questions please just ask! I'm here to help rather than say what you should/shouldn't be doing, I'll always respond by providing tested functioning code

import Tkinter as tk  
import time
import threading

switch = True
root = tk.Tk()

def blink():
 def run():
  while (switch == True):
   print('BLINK...BLINK...')
   time.sleep(0.5)
   if switch == False:
    break
 thread = threading.Thread(target=run)
 thread.start()

def switchon():  
 global switch
 switch = True
 print 'switch on' 
 blink()  
      
def switchoff():  
 print 'switch off'
 global switch
 switch = False    
      
def kill():  
 root.destroy()  
      
onbutton = tk.Button(root, text = "Blink ON", command = switchon)  
onbutton.pack()  
offbutton =  tk.Button(root, text = "Blink OFF", command = switchoff)  
offbutton.pack()  
killbutton = tk.Button(root, text = "EXIT", command = kill)  
killbutton.pack()  
      
root.mainloop()

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RE: How to break out of a while True: loop with a button

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Great! That should work.

One little suggestion: Move the "blink ()" call to the main at the bottom. Then you start the blink thread only once. As I read it (with my limited python knowledge), you will now start a new thread each time the "start blinking" button is pressed. So after turning it on and off again 5 times, there will be five threads  doing the blinking. This gives 5 times the output in the current incantation and a messy 5 blinking attempts through each other once you blink a real led....

Furthermore, it would be neater to stop the thread in the kill function.

Those are some valid points which would need attention in other languages, Python does much of the dirty work for us so there isn't any need to be concerned.

The blink function needs to be called whenever the button is pressed which is why its in a function that the button press calls. Placing it at the bottom will start the blinking but once the stop button is pressed, the start button will never doing anything.

Once the while loop breaks, the thread closes itself automatically through the Python subsystem and gets garbage collected.

If the start button is pressed again once the thread is actually running then additional threads will build up, this could be easily solved if it becomes a problem which is unlikely in a single user scenario. If it was something that I was going to release with a product then that would certainly be something that needs addressing, for now I think the Original Posters question has been answered and I'm happy to help with any future queries.

lucie tozer wrote:

 

Once the while loop breaks, the thread closes itself automatically through the Python subsystem and gets garbage collected.

Ooops. I was thinking of a mix of what you wrote and what I suggested. You're right! I mis-read what you coded. Sorry! Still, I'd prefer to have a "do the blinking" thread that is started once.

Interesting discussion.

Typically, I write a more simplified threaded procedure so that the straight-line code avoids embedded procedures (my preference or "style").  For whatever it is worth,

# Changes to original code on 2016-10-21 15:21 USA CT
# Simplified blink() procedure for future enhancement
# Moved threading references to main code section
# Doubled sleep time (not significant)
# Renamed "switch" to "flag_blinking" (not significant)
# Introduced flag_exiting

import Tkinter as tk    
import time  
import threading  
  
flag_blinking = False 
flag_exiting = False 

root = tk.Tk()  
  
def blink():  
    global flag_blinking  
    global flag_exiting   
    while ( True ):
        if (flag_blinking == True):  
            print('BLINK...BLINK...') 
        if (flag_exiting == True):
            return 
        time.sleep(1.0)  

def switchon():    
    global flag_blinking  
    flag_blinking = True  
    print 'flag_blinking on'   
        
def switchoff():    
    global flag_blinking  
    flag_blinking = False      
    print 'flag_blinking off'  
        
def kill(): 
    global flag_exiting   
    flag_exiting = True
    root.destroy()    
        
thread = threading.Thread(target=blink)  
thread.start()  

onbutton = tk.Button(root, text = "Blink ON", command = switchon)    
onbutton.pack()    
offbutton =  tk.Button(root, text = "Blink OFF", command = switchoff)    
offbutton.pack()    
killbutton = tk.Button(root, text = "EXIT", command = kill)    
killbutton.pack()    
  
root.mainloop() 

Hey, another Toaru Majutsu fan?

Yes

ps. Its good to see somebody else's perspective followed up with a working example, shows how a persons coding is unique and individual to them as their fingerprints are!!

I tried to use the code that you supplied but I was unable to get it to work. Like before the interface locks up once the while loop was entered.

I was launching it within IDLE.

That was great, I was looking at different threading examples online but never found anything as simple as what you just presented. Really appreciate the example!