code: def set_event(self, ref, tk_event, callback):
          
          # has an EventCallback been created for this tk event
          if tk_event not in self._event_callbacks:
              self._event_callbacks[tk_event] = EventCallback(self._widget, self._tks, tk_event)
  
          # assign this ref to this event callback
          self._refs[ref] = self._event_callbacks[tk_event]
  
          # set up the callback
          self._refs[ref].set_callback(ref, callback)
docs: Sets a callback for this widget against a ref (reference) for a
          tk_event, setting the callback to None will remove it.