def rename(self, data, variables=None):
        """
        Renames the columns according the variable mapping.

        Parameters
        ----------
        data: DataFrame
        variables: None or dict, default None
            If None, uses self.variables

        Returns
        -------
        data: DataFrame
            Renamed data.
        """
        if variables is None:
            variables = self.variables
        return data.rename(columns={y: x for x, y in variables.items()})