def exception(self, timeout=None):

        with self._condition:
            if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]:
                raise CancelledError()
            elif self._state == FINISHED:
                return self._exception

            self._condition.wait(timeout)

            if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]:
                raise CancelledError()
            elif self._state == FINISHED:
                return self._exception
            else:
                raise TimeoutError()

    # The following methods should only be used by Executors and in tests.
Return the exception raised by the call that the future represents.
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/concurrent/concurrent/futures/_base.py/Future.exception