code: def event_key_pressed(self, event):
          
          char = event.char
          if not char:
              return
  
          if char in string.ascii_letters:
              char = invert_shift(char)
  
          self.user_input_queue.put(char)
  
          # Don't insert the char in text widget, because it will be echoed
          # back from the machine!
          return "break"
docs: So a "invert shift" for user inputs:
          Convert all lowercase letters to uppercase and vice versa.