def clear_ddata(self):
        """ Clear the working copy of data

        Harmless, as it preserves the reference copy and the treatment dict
        Use only to free some memory

        """
        self._ddata = dict.fromkeys(self._get_keys_ddata())
        self._ddata['uptodate'] = False
Clear the working copy of data

        Harmless, as it preserves the reference copy and the treatment dict
        Use only to free some memory