def get_value(self, variable = None, table = None):
        """
        Get value

        Parameters
        ----------
        variable : string
                  name of the variable
        table : string, default None
                name of the table hosting the variable
        Returns
        -------
        df : DataFrame, default None
             A DataFrame containing the variable
        """
        assert variable is not None, "A variable is needed"
        if table not in self.tables:
            log.error("Table {} is not found in survey tables".format(table))
        df = self.get_values([variable], table)
        return df
Get value

        Parameters
        ----------
        variable : string
                  name of the variable
        table : string, default None
                name of the table hosting the variable
        Returns
        -------
        df : DataFrame, default None
             A DataFrame containing the variable