How to correctly implement multithreading using a Tkinter GUI?

Question

I am trying to implement Multithreading while using a GUI in tkinter. I came along this solution, but i am failing to implement it.

So basically i need to know:

How do i need to alter my Code to make the Progressbar interact nice and smoothly, i.e. not going in unresponsive mode when the GUI loses focus?

This is my code boiled down:

from tkinter import *
import queue
import threading
from tkinter.ttk import *

class ThreadedTask(threading.Thread):
    def __init__(self, queue):
        threading.Thread.__init__(self)
        self.queue = queue  

# Gui class
class MyGui(Frame):
    def __init__(self, parent):
        super().__init__(parent)
        self.parent = parent
        self.queue = queue.Queue() 
        self.init_ui()

    # define a button to fulfill long task
    def init_ui(self):        
        self.frame = Frame(self, relief=RAISED, borderwidth=1)
        self.frame.grid(row=0, column=0, columnspan=1, sticky='ns')
        self.status_frame = Frame(self, relief=RAISED, borderwidth=1, height=20)
        self.status_frame.grid(row=1, column=0, columnspan=3, sticky='nesw')
        self.status_frame.grid_configure(padx=3, pady=3)
        self.button = Button(self.frame, text='do Stuff', command=self.do_stuff)
        self.button.grid(padx=3, pady=3)
        self.progress = Progressbar(self.status_frame, orient="horizontal", length=80, mode="determinate")
        self.progress.grid(row=1, column=0, pady=3, sticky='nesw')
        self.grid()

    # start ThreadedTask here 
    def do_stuff(self):
        ThreadedTask(self.queue).start()
        self.queue_process(DoStuffClass(ui=self))

    def queue_process(self, process, retry_time=10): 
        self.master.after(retry_time, process) 

    def update_status(self):
        self.parent.update_idletasks()

class DoStuffClass:
    def __init__(self, ui=None):
        self.ui = ui
        self.long_task()

    def long_task(self):
        # do stuff here and update the progressbar from MyGui
        self.ui.progress['maximum'] = 10000
        # some lengthy task ...
        for i in range(10000):
            print(i)
            self.ui.progress.step()
            self.ui.parent.update_idletasks()


# main
root = Tk()
root.geometry("150x80+50+50")
MyGui(root)
root.mainloop()
root.quit()

I think right now my problem is the wrong implementation of queues and Threading, i.e. self.queue_process(DoStuffClass(ui=self))... the behaviour is the same as if i wouldn't use a queue and Multithread at all. The progressbar works, as long as it stays "in focus", which means me not clicking anything else on the desktop. As soon as i'm clicking elsewhere on the desktop and the GUI loses focus, the GUI goes in "unresponsive" mode and the progress bar does not update anymore. Also, sometimes Tcl closes the wrong Thread, which makes the whole program crash.

You boiled-down your code too much. When I try to run it: AttributeError: 'MyGui' object has no attribute 'preview_button' from line self.preview_button.grid(padx=3, pady=3). BTW, the main thread is the one that is executed first by the Python interpreter. — martineau
– martineau, Commented Aug 28, 2019 at 15:39
You may find Freezing/Hanging tkinter Gui in waiting for the thread to complete helpful. — martineau
– martineau, Commented Aug 28, 2019 at 15:42
@martineau Hello Martineau, i've updated my Code. I tested it and it works until the line self.queue_process(DoStuffClass(ui=self)) which is the part i am uncertain with. However, you do see a progressbar moving while doing the lengthy task, and when u change focus of the minigui, it will go in unresponsive mode (and for my original program -- sometimes crash). — Ougaga
– Ougaga, Commented Aug 29, 2019 at 6:53
Your question is very broad and there are too many sub-questions (some of which make little sense — i.e. there's no such thing a sub-thread). The main thread is the first code the Python interpreter starts running. You can implement ThreadedTask class anywhere you want. Threads aren't spawned from it, instances of it are separate threads. The basic thing to keep in mind is that only one thread (usually the main one) should be updating the GUI (via after()). Other threads need to put stuff in Queues and the main thread should check them and update the GUI depending on what it finds. — martineau
– martineau, Commented Aug 29, 2019 at 17:51
You could also have a look at the .after() method (in Tkinter) that will allow you to run a function in Tkinter without pausing the GUI. This will also remove the need to use the threading module. — enchant97
– enchant97, Commented Aug 30, 2019 at 14:46

Ougaga · Accepted Answer · 2019-09-05 07:14:54Z

So after a couple of tries i figured out what to do:

from tkinter import *
import queue
import threading
from tkinter.ttk import *

class ThreadedTask(object):
    def __init__(self, parent):
        self.parent = parent        
        self.queue = queue.Queue()
        self.gui = MyGui(parent, self.queue)
        self.work_queue()

    def work_queue(self):
        """ Check every 100 ms if there is something new in the queue. """
        try:
            self.parent.after(200, self.work_queue)
            print('working queue with task {}...'.format(self.queue.get_nowait()))
        except queue.Empty:
            pass


# Gui class
class MyGui(Frame):
    def __init__(self, parent, queue):
        super().__init__(parent)
        self.parent = parent
        self.queue = queue
        self.init_ui()

    # define a button to fulfill long task
    def init_ui(self):   
        self.frame = Frame(self, relief=RAISED, borderwidth=1)
        self.frame.grid(row=0, column=0, columnspan=1, sticky='ns')
        self.status_frame = Frame(self, relief=RAISED, borderwidth=1, height=20)
        self.status_frame.grid(row=1, column=0, columnspan=3, sticky='nesw')
        self.status_frame.grid_configure(padx=3, pady=3)        
        self.button = Button(self.frame, text='do Stuff', command=self.init_button_loop)
        self.button.grid(padx=3, pady=3)
        self.progress = Progressbar(self.status_frame, orient="horizontal", length=80, mode="determinate")
        self.progress.grid(row=1, column=0, pady=3, sticky='nesw')
        self.grid()


    def start_thread(self, function_name, queue):
        t = threading.Thread(target=function_name, args=(queue,))
        # close thread automatically after finishing task
        t.setDaemon(True)
        t.start()

    # execute button push by spawning a new thread
    def init_button_loop(self):
        self.start_thread(self.exec_button_loop, self.queue)    

    # execute long task
    def exec_button_loop(self, queue):
        self.progress['maximum'] = 10000
        for i in range(10000):
            # update progressbar
            queue.put(self.progress.step())


# main
root = Tk()
root.geometry("150x80+50+50")
client = ThreadedTask(root)
root.mainloop()

The Difficulty was to figure out how to interact with queues and threads, while pressing buttons in the gui.

Basically my fault was to declare the queue in the wrong class and the wrong use of the after function while not knowing where to start the threads.

The new implementation follows the principle of filling the queue up in a new thread, which is spawned when pressing the gui button. The queue is periodically checked from the main thread whether theres something to do. This prevents unresponsiveness due to a safe communication between gui and mainthread.

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