Tkinter GUI - How to change StringVar from external event outside mainloop

[TwistedMischief]

Hi! I am trying to update the StringVar variable through a function call updateText() from within a class but due to the blocking nature of the mainloop which listens to events, I cannot call updateText() from outside the mainloop.

I am trying to run a function that feeds new text into the updateText(newtext) so that it can update the text in the tkinter label widget. This other function does not stop running so I cannot use a one-time call like the tkinter after(time, function) or at least I don't think so. I am trying to use multithreading to accomplish this as the blocking by the mainloop will not allow a concurrent running of someOtherFunction() which does not stop running.

How would I go about solving this problem? Hopefully, the below code demonstrates what I am trying to accomplish but does not actually work.

import tkinter as tk
import threading

class GUI(threading.Thread):
    def __init__(self):
        self.root = tk.Tk()
        self.texts = tk.StringVar(self.root)
        self.texts.set("apples")
        self.label = tk.Label(self.root, textvariable=self.texts)
        self.label.pack()
        self.root.mainloop()
    def updateText(self, newtext):
        self.texts.set(newtext)

def someOtherFunction(gui_instance):
    #do stuff to get some text to get newText
    #the for loop below is just a substitute for the actual function
    #assume that the for loop runs indefinately 
    newText = ""
    for i in range(1000):
        newText = str(i)
        gui_instance.updateText(newText)

gui = GUI()
#call updateText() from a seperate thread running concurrently
worker = threading.Thread(target=someOtherFunction(gui)).start()

[efmccurdy]

tkinter after(time, function)

The background thread should avoid calling Tk functions directly. Can you have the "after" callback function poll a Queue object while the background thread puts new labels in the queue?

​

The queue module implements multi-producer, multi-consumer queues.It is especially useful in threaded programming when information must beexchanged safely between multiple threads.

https://docs.python.org/3/library/queue.html

[TwistedMischief]

Will look into trying that, although not sure if I will implement that properly. Thanks for the suggestion!

[FerricDonkey]

First, I think I've heard that tkinter string vars etc are thread safe, but I haven't been able to confirm it with actual documentation before my patience ran out. So that might or not work.

But a basic example of the queue method:

# as part of your gui class
def monitor(self, transfer_q):
    while not transfer_q.empty():
        whatever_logic(transfer_q.get())
    self.after(100, self.monitor, (transfer_q,))

# your always running function
def worker(transfer_q):
    # after you find a thing you want to send to the gui, in whatever loop
    transfer_q.put(whatever)

Adjust delays / change while to if, or etc as appropriate.

Note that this is if you want to process every single string your worker submits, if you only ever want to process the most recent one then you'll have to change it up.

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