Python 3 and Tkinter: progress bar does not update during script execution

Question

Hello. I am creating a layout converter using Python 3.6.4 and Tkinter.

Among other things the GUI must have a progress bar that updates its value with each interaction of the conversion process, for example, with each processed line of the file being converted, the progress bar must be updated.

The problem I am facing is that the "program" hangs while running the conversion and the bar is not updated. The bar only updates at the end of the conversion operation.

The basic part of the script is this (I have removed everything that is not relevant and left only the essential to see where the problem is or where I am missing):

import sys
from time import sleep
from tkinter import *
from tkinter.ttk import Progressbar

class Gui:

    def __init__(self):
        self.Window = Tk()
        self.Window.geometry('{0}x{1}'.format(600, 400))

        self.progress = StringVar()
        self.progress.set(0)
        self.progressBar = Progressbar(self.Window, maximum=100, orient=HORIZONTAL, variable=self.progress)
        self.progressBar.grid(row=0, column=0, sticky=W + E)

        self.startButton = Button(self.Window, text='Iniciar', command=self.start)
        self.startButton.grid(row=0, column=2)

    def start(self):
        for t in range(0,100):
            self.progress.set(t)
            sleep(0.1)

    def run(self):
        self.Window.mainloop()
        return 0

if __name__ == '__main__':
    Gui = Gui()
    sys.exit(Gui.run())

I’m a beginner in Python but I have long experience (fifteen years) with PHP and etc.

Answer 1

The problem you are having is that Tkinter (as well as other graphical toolkits in various languages such as GTK, Qt, etc...) run in a single thread. A graphical program always has a main loop, which is where the framework has the code to collect all the events that arrive at the application, and call the corresponding functions so that they are processed.

Hence you configure your application for when the button is pressed, the method start should be called - and passes control to the mainloop tkinter. When the button is pressed, Tkinter takes the mouse event from the operating system, creates the internal event, and calls the start function -- and this in turn does not return control for Tkinter. Instead it makes 100 updates on self.progress, and only after that returns control. When the start has just run is that Tkinter will process all its internal events from set of progressibar.

Even more, every call to time.sleep, your program stands still, not responding to external events or anything - because the code that does this is in the main loop.

And how to tidy up? Simple: in no graphic program you call the tine.sleep or equivalent function (unless it is integrated into the framework). In the case of Tkinter, the correct one is to call the Window.after saying a method to be called after as many milliseconds. You do this scheduling, and return control to the main loop of Tkinter. Of course the status of the progress bar should be stored in some lgar between a chassis in another - as you are already within a class, just use a specific attribute for this.

class Gui:

    def __init__(self):
        ...
        self.progress_count = 0


    def start(self):
        self.progress.set(self.progress_count)
        self.progress_count += 1
        if self.progress_count < 100:
             self.Window.after(100, self.start)

Answer 2

After hours of searching last night, I found something that also solved the problem: using self.Window.update_idletasks() inside the loop.

Calls all pending Idle tasks, without Processing any other Events. This can be used to carry out Geometry management and redraw widgets if necessary, without Calling any callbacks.

So the final code (with a few minor adjustments) was like this

class Gui:

    def __init__(self):
        self.Window = Tk()
        self.Window.geometry('{0}x{1}'.format(600, 400))

        self.progress = DoubleVar()
        self.progress.set(0)
        self.progressBar = Progressbar(self.Window, maximum=100, orient=HORIZONTAL, variable=self.progress)
        self.progressBar.grid(row=0, column=0, sticky=W + E)

        self.startButton = Button(self.Window, text='Iniciar', command=self.start)
        self.startButton.grid(row=0, column=2)

    def start(self):
        for t in range(0, 100, 20):
            self.progress.set(t)
            print(t)
            self.Window.update_idletasks()
            sleep(2)
        self.progress.set(100)
        print('Fim...')

    def run(self):
        self.Window.mainloop()
        return 0

if __name__ == '__main__':
    Gui = Gui()
    sys.exit(Gui.run())