def event_key_pressed(self, event):
        """
        So a "invert shift" for user inputs:
        Convert all lowercase letters to uppercase and vice versa.
        """
        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"