def show_options(self, key=""):
        """Returns a copy the options :class:`~pandas.DataFrame`.

        Called on jupyter notebook, it will print them in pretty
        :class:`~pandas.DataFrame` format.

        :param str key: First identifier of the option. If not provided,
            all options are returned.

        :return: :class:`~pandas.DataFrame`
        """
        key, _ = _lower_keys(key, '')

        if key == "":
            return self.gc.copy()
        return self.gc[(self.gc["k1"] == key)].copy()