code: def reset_state(self):
          
          assert not self._reset_done, "reset_state() was called twice! This violates the API of DataFlow!"
          self._reset_done = True
  
          # __del__ not guaranteed to get called at exit
          atexit.register(del_weakref, weakref.ref(self))
docs: All forked dataflows should only be reset **once and only once** in spawned processes.
          Subclasses should call this method with super.