def get_option_alternatives(self, key, subkey):
key, subkey = _lower_keys(key, subkey)<EOL>_entry_must_exist(self.gc, key, subkey)<EOL>return self.gc[(self.gc["<STR_LIT>"] == key) &<EOL>(self.gc["<STR_LIT>"] == subkey)]["<STR_LIT>"].values[<NUM_LIT:0>]<EOL>
Get list of available values for an option.

        :param str key: First identifier of the option.
        :param str subkey: Second identifier of the option.

        :return: Union[:func:`list`, :data:`None`] - alternative values
            for the option, if any specified (otherwise, is open).

        :raise:
            :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                option.