<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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