code: def number(self) -> Optional[Union[str, Set[str]]]:
          
          if "number" not in self.data.columns:
              return None
          if all(self.data.number.isna()):
              return None
          tmp = set(self.data.number)
          if len(tmp) == 1:
              return tmp.pop()
          logging.warn("Several numbers for one flight, consider splitting")
          return tmp
docs: Returns the unique number value(s) of the DataFrame.