code: def _set_result(self, result):
          
          if self._result_set.isSet():
              return
          self._value = result
          self._result_set.set()
docs: Set the result of the EventualResult, if not already set.
  
          This can only happen in the reactor thread, either as a result of
          Deferred firing, or as a result of ResultRegistry.stop(). So, no need
          for thread-safety.