code: def set_exception(self, exception):
          
          # Sanity check: A future can only complete once.
          if self.done():
              raise RuntimeError("set_exception can only be called once.")
  
          # Set the exception and trigger the future.
          self._exception = exception
          self._trigger()
docs: Set the result of the future to the given exception.
  
          Args:
              exception (:exc:`Exception`): The exception raised.