code: def _clean(self):
          
          if self._workers:
              for url in self._workers.copy():
                  w = self._workers[url]
                  if w.is_finished():
                      self._workers.pop(url)
                      self._paths.pop(url)
                      if url in self._get_requests:
                          self._get_requests.pop(url)
  
          else:
              self._timer.stop()
docs: Check for inactive workers and remove their references.