tw1.py

from twisted.internet import defer, reactor

def stopping(result):
    print('got', result)
    # raise RuntimeError('fail')
    reactor.stop()
    return result

def raiseException(_):
    raise Exception()

def handleError():
    pass

def printIt(result):
    print('----' * 2)
    def done(_):
        print('done')
    d = defer.Deferred()
    d.addCallback(done)
    reactor.callLater(1, d.callback, True)
    # return d

if __name__ == '__main__':

    d = defer.Deferred()
    # d.addCallback(raiseException)
    d.addCallback(printIt)
    # d.addErrback(handleError)
    d.addBoth(stopping)

    reactor.callLater(1, d.callback, True)

    print('start')
    reactor.run()
    print('stop')