code: def _prepare_io_handler_cb(self, handler):
          
          self._anything_done = True
          logger.debug("_prepar_io_handler_cb called for {0!r}".format(handler))
          self._configure_io_handler(handler)
          self._prepare_sources.pop(handler, None)
          return False
docs: Timeout callback called to try prepare an IOHandler again.