How to update labels in tkinter dynamically?

Question

I've created this code that updates a label every second to indicate that something is loading (run the code to see what I mean). I'm using the threading module with tkinter but I feel like there must be a more efficient way to do this.

Here is my code:

from tkinter import *
from time import sleep
import threading

root = Tk()
new_var = StringVar()
new_var.set('Loading')

def change_text():
    array = [".", "..", "...", ""]
    while True:
        for num in range(4):
            sleep(1)
            new_var.set(f"Loading{array[num]}")
            root.update_idletasks()

l = Label(root, textvariable = new_var)
l.pack()

Loading_animation = threading.Thread(target=change_text)
Loading_animation.start()
root.mainloop()

Also, if there isn't a better way to do this how do I prevent the error that I keep receiving whenever I close the root window?

Thank you!

@IgorS this doesn't address the underlying error. I believe the underlying error is that I haven't rejoined the second thread that I created, but I don't know how to do that since my code is stuck in the "root.mainloop()" at the end. — Matt
– Matt, Commented Jul 13, 2018 at 21:31

Mike - SMT · Accepted Answer · 2018-07-14 00:28:57Z

2

Here is a simpler method that doesn't involve threading.

Keep a counter and every second call the function. In the function simply set the text to each item in the list by the counter as an index.

Update: To answer your question in the comments.

This will not get stuck in some loop that stops us from reaching the mainloop() because this code only adds a command to be run on the event list at a regular interval of 1 second. What is actually happening is the after() method will add a new even to run no sooner than 1 second (1000 milliseconds). Because Tkinter is event-driven Tkinter will handle each even in the list as it comes after every mainloop() cycle.

import tkinter as tk

root = tk.Tk()
counter = 0

def change_text():
    global counter
    my_list = [".", "..", "...", ""]
    if counter != 3:
        l.config(text="Loading{}".format(my_list[counter]))
        counter += 1
        root.after(1000, change_text)
    else:
        l.config(text="Loading{}".format(my_list[counter]))
        counter = 0
        root.after(1000, change_text)

l = tk.Label(root, text = "")
l.pack()

change_text()
root.mainloop()

edited Jul 14, 2018 at 0:28

answered Jul 13, 2018 at 21:58

Mike - SMT

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1 Comment

Matt Over a year ago

Thanks Mike the code works exactly as I'd like, but there's one thing I don't understand. Shouldn't the code be stuck in the change_text() loop and therefore be unable to execute root.mainloop() to create the window.

Novel · Accepted Answer · 2018-07-13 23:16:53Z

2

Here is the same answer as @Mike-SMT, but using the cycle function to make it a lot neater.

import tkinter as tk
from itertools import cycle

root = tk.Tk()
my_list = cycle([".", "..", "...", ""])

def change_text():
    l.config(text="Loading{}".format(next(my_list)))
    root.after(1000, change_text)

l = tk.Label(root)
l.pack()

change_text()
root.mainloop()

answered Jul 13, 2018 at 23:16

Novel

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4 Comments

Matt Over a year ago

I have the same question for you as I did Mike - how is the script able to execute root.mainloop() as shouldn't it be stuck in the infinite change_test() loop?

Novel Over a year ago

The mainloop is an infinite loop that constantly checks if an event has happened that it needs to respond to. It also checks a list of scheduled events to see if it's time to do one of those. The after function IS NOT a loop, all it does is add an event to the scheduled events list, in your case it adds "at current_time + 1000 run the change_test function". Then it returns. It does not block like sleep does. When the mainloop sees that the time is greater than the scheduled time, it runs change_text, which of course causes another addition to the event schedule.

Novel Over a year ago

TLDR: after is not a loop, but it modifies the mainloop, so we can use it as if it were a loop.

Mike - SMT Over a year ago

@Novel I like this. I was aware of the itertools however never had a need for them thus far and so it didn't cross my mind.

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