docs: def open(self):
          """Start. Multiple calls have no effect.
  
          Not safe to call from multiple threads at once.
          """
          with self._lock:
              if self._thread_will_exit:
                  # If the background thread has read self._stopped as True
                  # there is a chance that it has not yet exited. The call to
                  # join should not block indefinitely because there is no
                  # other work done outside the while loop in self._run.
                  try:
                      self._thread.join()
                  except ReferenceError:
                      # Thread terminated.
                      pass
              self._thread_will_exit = False
              self._stopped = False
          started = False
          try:
              started = self._thread and self._thread.is_alive()
          except ReferenceError:
              # Thread terminated.
              pass
  
          if not started:
              thread = threading.Thread(target=self._run, name=self._name)
              thread.daemon = True
              self._thread = weakref.proxy(thread)
              _register_executor(self)
              thread.start()