<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def num_open_files(self):
 """Amount of opened files in the system"""

        return reduce(lambda x, y: x + y, (1 for rlist in self._fdict.values() if len(rlist) > 0), 0)