python - Tkinter: Updating progressbar when a function is called

Question

Imagine the following simple example:

def doNothing():
  sleep(0.5)
  barVar.set(10)
  sleep(0.5)
  barVar.set(20)
  sleep(0.5)
  barVar.set(30)

mainWindow = Tk()
barVar = DoubleVar()
barVar.set(0)
bar = Progressbar(mainWindow, length=200, style='black.Horizontal.TProgressbar', variable=barVar, mode='determinate')
bar.grid(row=1, column=0)
button= Button(mainWindow, text='Click', command=doNothing)
button.grid(row=0, column=0)
mainWindow.mainloop()

What I get when I run this, the progressbar is already at 30% when clicking the button, no progress in front of me. Like attached:

What I need: I can see the progress in front of me (not hanging then suddenly 30%)

Update: I upadted the code according to @Bernhard answer, but still I can not see the progress in front of me. Just a sudden jump of 30% after waiting 1.5 sec

Seocnd Update: I'm only using sleep here as a simulation for a process that takes time, like connecting over ssh and grabing some info.

Answer 1

Do not use sleep() in tkinter. The entire reason for you problem is sleep() will freeze tkinter until it is done with its count so what you are seeing is a frozen program and when the program is finally released its already set to 30 percent on the next mainloop update.

Instead we need to use Tkinter's built in method called after() as after is specifically for this purpose.

import tkinter as tk import tkinter.ttk as ttk

mainWindow = tk.Tk()

def update_progress_bar():
    x = barVar.get()
    if x < 100:
        barVar.set(x+10)
        mainWindow.after(500, update_progress_bar)
    else:
        print("Complete")


barVar = tk.DoubleVar()
barVar.set(0)
bar = ttk.Progressbar(mainWindow, length=200, style='black.Horizontal.TProgressbar', variable=barVar, mode='determinate')
bar.grid(row=1, column=0)
button= tk.Button(mainWindow, text='Click', command=update_progress_bar)
button.grid(row=0, column=0)

mainWindow.mainloop()

If you want the bar to appear to move smoothly you will need to speed up the function call and reduce the addition to the DoubbleVar.

import tkinter as tk
import tkinter.ttk as ttk


mainWindow = tk.Tk()

def update_progress_bar():
    x = barVar.get()
    if x < 100:
        barVar.set(x+0.5)
        mainWindow.after(50, update_progress_bar)
    else:
        print("Complete")


barVar = tk.DoubleVar()
barVar.set(0)
bar = ttk.Progressbar(mainWindow, length=200, style='black.Horizontal.TProgressbar', variable=barVar, mode='determinate')
bar.grid(row=1, column=0)
button= tk.Button(mainWindow, text='Click', command=update_progress_bar)
button.grid(row=0, column=0)

mainWindow.mainloop()