code: def _process_event(self, key, mask):
          
          self._logger.debug('Processing event with key {} and mask {}'.format(key, mask))
          fileobj, (reader, writer) = key.fileobj, key.data
          if mask & selectors.EVENT_READ and reader is not None:
              if reader._cancelled:
                  self.remove_reader(fileobj)
              else:
                  self._logger.debug('Invoking reader callback: {}'.format(reader))
                  reader._run()
          if mask & selectors.EVENT_WRITE and writer is not None:
              if writer._cancelled:
                  self.remove_writer(fileobj)
              else:
                  self._logger.debug('Invoking writer callback: {}'.format(writer))
                  writer._run()
docs: Selector has delivered us an event.