def allowed_values(self):
        """A tuple containing the allowed values for this Slot.

        The Python equivalent of the CLIPS slot-allowed-values function.

        """
        data = clips.data.DataObject(self._env)

        lib.EnvSlotAllowedValues(
            self._env, self._cls, self._name, data.byref)

        return tuple(data.value) if isinstance(data.value, list) else ()