<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _refresh(self):
 """ refresh internal directory cache """

        log.debug('refreshing directory cache')
        self._users.update(list(self._user_gen()))
        self._channels.update(list(self._channel_gen()))