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

OK This is what I thought I was being asked to do with regards to coding this without using threads, but this does not work either.

import tkinter as tk
import time

root = tk.Tk()
switch = False

def blink(switch):
    while switch == True:
        print('BLINK...BLINK...')
        time.sleep(0.5)
    while switch == False:
        break

def switchon():
    switch = True
    blink(switch)

def switchoff():
    switch = False
    blink(switch)

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()

OK This is what I thought I was being asked to do with regards to coding this without using threads, but this does not work either.

import tkinter as tk
import time

root = tk.Tk()
switch = False

def blink(switch):
    while switch == True:
        print('BLINK...BLINK...')
        time.sleep(0.5)
    while switch == False:
        break

def switchon():
    switch = True
    blink(switch)

def switchoff():
    switch = False
    blink(switch)

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()

Sean,

The "use threads" suggestion was a "future improvement" because it "might improve the performance". I tried to say that "improving performance" at this stage is NOT a good idea for two reasons. 1) I don't think it actually improves performance and 2) is detracts from the issue of trying to get things to work.

I am not very familiar with the language you're using. But the way you've written the blink function is never going to work.

You are calling the blink function, with either a true or a false argument. In the first case (true) the while switch==true will ALLWAYS be true and remain true, so the loop can never end. In the second case (false), when you call the function the while loop will simply never execute and the second while loop will issue a useless "break" function.

IF you get threads to work, and IF they work in python the way they work in C, then....

[code]

def blink:

    while True:

         if (switch) print ('blink')

         time.sleep (0.5)

[/code]

Now YOU have to figure out how to make this a thread. (I googled this for you and it is too complicated for me to understand in 5 minutes).  This function will continue to run, even when switch is false, but then it won't do the blink. 

You may have to do some python magic to make the "switch" variable a global variable.

Now when your program starts you do create_thread (blink) (however that is written in python). When the "start blinking" button is pressed you simply set switch=True, and when stop blinking button is pressed you simply do switch=False;

When "quit" is pressed you'd better somehow "kill" the thread that does the blinking.

Another way to implement this is to simply CREATE the thread doing the blinking when the start button is pressed and to stop the thread when either the stop or quit button is pressed.

That code doesn't work very well for a responsive interface, you would need to be holding the button down throughout the sleep cycle in order for it to be tested when the if statement comes back around (for how long depends on when in the sleep cycle you started pressing the button. The worst case scenario would be pressing it for 0.5s which is a lot longer than anybody would like out of an on screen button. Also the light would only blink if the button was being held down). Not to mention that keeping the loop going when its not needed to blink the button is wholly inefficient and im sure you can agree that is just plain bad practice.

The code I provided above is a direct drag and drop replacement for the original that doesnt use threading, im not going to say its the most efficient but it is working code that I tested before I posted and easy to read (for the sake of argument the performance increase of threading comes from not locking up a linux process and allowing it to be free'd up to do something else during the python threads sleep time, like draw the button to the screen efficiently, handle its file system, draw the desktop, send/recieve network data. All things that would benefit, when I talk about these things Im looking from an overall system advantage not just a narrow view of a single python script)

There are a few redundant parts used here, try:

    import tkinter as tk 

    import time 

    root = tk.Tk() 

    switch = False 

    def blink(): 

        while switch == True: 

            print('BLINK...BLINK...') 

            time.sleep(0.5)   

    def switchon(): 

        switch = True 

        blink() 

    def switchoff(): 

        switch = False 

        blink() 

    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()

------------------------------------------------------------------------------------------------------

Theres no need to add the switch into the function definition arguments, because its already being declared outside the functions its already globally used in the script and is causing confusion when declaring blink() function.

When I get chance later this afternoon I'll recreate a working example again and post the entire script rather than just a snippet so you can see it working. Where about are you looking for the "blink.....blink" text to be printed? It should come out in the console you used to launch your script (if you launch it without using a console/terminal you probably wont ever see the text)

My original test program also had the tkinter buttons in a thread so they were working independently. There is a very good chance that without using a thread at least once you will never be able to break out of the while loop in a responsive manner. Would you mind if I provided an example using threads? its really just going to be about 4 extra lines, with one of them being the import threads statement and is easy to understand. It would make your task so much simpler.

For a responsive buttons without any threading you are going to have to add a for loop which contains a sleep function for a miniscule amount of time and a call to yield within the loop, then another master loop to control it all. Without using threads the code is going to need about 20 more lines to achieve this task

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.

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.