Returns the list of column names that this index will be expecting as \
        inputs when it is called.
        
        :return     [<str>, ..]

def columns(self):
        """
        Returns the list of column names that this index will be expecting as \
        inputs when it is called.
        
        :return     [<str>, ..]
        """
        schema = self.schema()
        return [schema.column(col) for col in self.__columns]