code: def save_to_disk(self, filename_pattern=None):
      
      if not self._converter:
        raise RuntimeError(
            'Must set _converter on subclass or via set_converter before calling '
            'save_to_disk.')
  
      pattern = filename_pattern or self._default_filename_pattern
      if not pattern:
        raise RuntimeError(
            'Must specify provide a filename_pattern or set a '
            '_default_filename_pattern on subclass.')
  
      def save_to_disk_callback(test_record_obj):
        proto = self._convert(test_record_obj)
        output_to_file = callbacks.OutputToFile(pattern)
  
        with output_to_file.open_output_file(test_record_obj) as outfile:
          outfile.write(proto.SerializeToString())
  
      return save_to_disk_callback
docs: Returns a callback to convert test record to proto and save to disk.