code: def get_expected_update_frequency(self):
          # type: () -> Optional[str]
          
          days = self.data.get('data_update_frequency', None)
          if days:
              return Dataset.transform_update_frequency(days)
          else:
              return None
docs: Get expected update frequency (in textual rather than numeric form)
  
          Returns:
              Optional[str]: Update frequency in textual form or None if the update frequency doesn't exist or is blank.