def _refresh_dag_dir(self):
        """
        
        """
        elapsed_time_since_refresh = (timezone.utcnow() -
                                      self.last_dag_dir_refresh_time).total_seconds()
        if elapsed_time_since_refresh > self.dag_dir_list_interval:
            # Build up a list of Python files that could contain DAGs
            self.log.info("Searching for files in %s", self._dag_directory)
            self._file_paths = list_py_file_paths(self._dag_directory)
            self.last_dag_dir_refresh_time = timezone.utcnow()
            self.log.info("There are %s files in %s", len(self._file_paths), self._dag_directory)
            self.set_file_paths(self._file_paths)

            try:
                self.log.debug("Removing old import errors")
                self.clear_nonexistent_import_errors()
            except Exception:
                self.log.exception("Error removing old import errors")