code: def on_back_pressed(self):
          
          try:
              event = {'handled': False}
              self.back_pressed(event)
              return bool(event.get('handled', False))
          except Exception as e:
              #: Must return a boolean or we will cause android to abort
              return False
docs: Fire the `back_pressed` event with a dictionary with a 'handled'
          key when the back hardware button is pressed
          
          If 'handled' is set to any value that evaluates to True the
          default event implementation will be ignored.