def remove_done_callback(self, func):
        """Remove a callback from the long running operation.

        :param callable func: The function to be removed from the callbacks.
        :raises: ValueError if the long running operation has already
         completed.
        """
        if self._done is None or self._done.is_set():
            raise ValueError("Process is complete.")
        self._callbacks = [c for c in self._callbacks if c != func]
Remove a callback from the long running operation.

        :param callable func: The function to be removed from the callbacks.
        :raises: ValueError if the long running operation has already
         completed.