code: def add_done_callback(self, func):
          # type: (Callable) -> None
          
          # Still use "_done" and not "done", since CBs are executed inside the thread.
          if self._done is None or self._done.is_set():
              func(self._polling_method)
          # Let's add them still, for consistency (if you wish to access to it for some reasons)
          self._callbacks.append(func)
docs: Add callback function to be run once the long running operation
          has completed - regardless of the status of the operation.
  
          :param callable func: Callback function that takes at least one
           argument, a completed LongRunningOperation.