def number(self) -> Optional[Union[str, Set[str]]]:
        """Returns the unique number value(s) of the DataFrame."""
        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
Returns the unique number value(s) of the DataFrame.