code: def flight_id(self) -> Optional[str]:
          
          if "flight_id" not in self.data.columns:
              return None
          tmp = set(self.data.flight_id)
          if len(tmp) != 1:
              logging.warn("Several ids for one flight, consider splitting")
          return tmp.pop()
docs: Returns the unique flight_id value(s) of the DataFrame.
  
          If you know how to split flights, you may want to append such a column
          in the DataFrame.