title: Python3, Tkinter - display message after process completion
body: <p>I'm using python 3 and tkinter. I have a simple GUI that triggers a back-end process (in a different class, instantiated in the GUI class). I want to display a message to the user once the back-end process is complete, i.e. use messagebox (or anything else, doesn't really matter) but only after the process is complete.
  How can I do that?</p>
  
  <p>Thanks in advance</p>