Returns a `data_frame.columns`.
        :param usage (str): should be a value from [ALL, INCLUDE, EXCLUDE].
                            this value only makes sense if attr `columns` is also set.
                            otherwise, should be used with default value ALL.
        :param columns: * if `usage` is all, this value is not used.
                        * if `usage` is INCLUDE, the `df` is restricted to the intersection
                          between `columns` and the `df.columns`
                        * if usage is EXCLUDE, returns the `df.columns` excluding these `columns`
        :return: `data_frame` columns, excluding `target_column` and `id_column` if given.
                 `data_frame` columns, including/excluding the `columns` depending on `usage`.

def get_columns(self, usage, columns=None):
        """
        Returns a `data_frame.columns`.
        :param usage (str): should be a value from [ALL, INCLUDE, EXCLUDE].
                            this value only makes sense if attr `columns` is also set.
                            otherwise, should be used with default value ALL.
        :param columns: * if `usage` is all, this value is not used.
                        * if `usage` is INCLUDE, the `df` is restricted to the intersection
                          between `columns` and the `df.columns`
                        * if usage is EXCLUDE, returns the `df.columns` excluding these `columns`
        :return: `data_frame` columns, excluding `target_column` and `id_column` if given.
                 `data_frame` columns, including/excluding the `columns` depending on `usage`.
        """
        columns_excluded = pd.Index([])
        columns_included = self.df.columns

        if self.has_target():
            columns_excluded = pd.Index([self.target_column])

        if self.has_id():
            columns_excluded = columns_excluded.union(pd.Index([self.id_column]))

        if usage == self.INCLUDE:
            try:
                columns_included = columns_included.intersection(pd.Index(columns))
            except TypeError:
                pass
        elif usage == self.EXCLUDE:
            try:
                columns_excluded = columns_excluded.union(pd.Index(columns))
            except TypeError:
                pass

        columns_included = columns_included.difference(columns_excluded)
        return columns_included.intersection(self.df.columns)