title: Tkinter GUI with progress bar
body: <p>I have a simple Tk GUI and a long process in a function attached to a button. I want a progress bar when I click on the button, just like it starts a long process.</p>
  <p>How can I do that? This is my current code:</p>
  <pre><code>from tkinter import Button, Tk, HORIZONTAL
  
  from tkinter.ttk import Progressbar
  import time
  
  
  class MonApp(Tk):
      def __init__(self):
          super().__init__()
  
          bt1 = Button(self, text='Traitement', command=self.traitement)
          bt1.grid()
          self.progress = Progressbar(self, orient=HORIZONTAL, length=100, mode='indeterminate')
          self.progress.grid()
          self.progress.grid_forget()
  
      def traitement(self):
          self.progress.grid()
          self.progress.start()
          time.sleep(15) 
          ## Just like you have many, many code lines...
  
          self.progress.stop()
  
  
  if __name__ == '__main__':
      app = MonApp()
      app.mainloop()
  </code></pre>
  <p>How can I put a progress bar in that application?</p>