tkinter gui with progress bar

For all the GUI elements to modify themselves (in your case, for the progress bar to move) the execution must hit app.mainloop().

In your case, def traitement(self): starts and then stops the progressbar without hitting the mainloop, so it fails to visibly reflect the intended progressbar movement on the GUI. The catch here is, when the execution hits mainloop, progressbar is configured to 'stop' state.

Hence, it is a good idea to execute time consuming activities on a different Thread as shown by @xmcp

However, if you DO NOT want to use threading, you can use the after method to achieve what you want:

def stop_progressbar(self):
    self.progress.stop()

def traitement(self):
    self.progress.grid()
    self.progress.start()
    self.after(15000, self.stop_progressbar) 
    ## Call Just like you have many, many code lines...

The above code used self.after() method which will execute the stop_progressbar method to stop after 15 seconds, instead of time.sleep() which blocks the mainthread.

You can find ttk.Progressbar at tkdocs

import time
from tkinter import *
from tkinter.ttk import *

tk = Tk()
progress = Progressbar(tk, orient=HORIZONTAL, length=100, mode='determinate')


def bar():
    progress['value'] = 20
    tk.update_idletasks()
    time.sleep(1)
    progress['value'] = 50
    tk.update_idletasks()
    time.sleep(1)
    progress['value'] = 80
    tk.update_idletasks()
    time.sleep(1)
    progress['value'] = 100

progress.pack()
Button(tk, text='foo', command=bar).pack()
mainloop()

It's better to use threading and run your code in another thread.

Like this:

import threading
import time
from tkinter import Button, Tk, HORIZONTAL
from tkinter.ttk import Progressbar

class MonApp(Tk):
    def __init__(self):
        super().__init__()

        self.btn = Button(self, text='Traitement', command=self.traitement)
        self.btn.grid(row=0, column=0)
        self.progress = Progressbar(self, orient=HORIZONTAL, length=100, mode='indeterminate')

    def traitement(self):
        def real_traitement():
            self.progress.grid(row=1,column=0)
            self.progress.start()
            time.sleep(5)
            self.progress.stop()
            self.progress.grid_forget()

            self.btn['state']='normal'

        self.btn['state']='disabled'
        threading.Thread(target=real_traitement).start()


if __name__ == '__main__':
    app = MonApp()
    app.mainloop()