如何從延遲的返回實例中獲取價值

Question

我使用txmongo lib作為mongoDB的驅動程序。 在其有限的文檔中，txmongo中的find函數將返回deferred的實例，但是如何獲得實際結果（如{“ IP”：11.12.59.119}）？ 我嘗試了yield，str（）和repr（），但是沒有用。

def checkResource(self, resource):
    """ use the message to inquire database
        then set the result to a ip variable
    """
    d = self.units.find({'$and': [{'baseIP':resource},{'status':'free'}]},limit=1,fields={'_id':False,'baseIP':True})
    #Here above, how can I retrieve the result in this deferred instance??
    d.addCallback(self.handleReturnedValue)
    d.addErrback(log.err)
    return d

def handleReturnedValue(self, returned):
    for ip in returned:
        if ip is not None:
            d = self.updateData(ip,'busy')
            return d
        else:
            return "NA"

Answer 1

如果要以扭曲的方式編寫異步代碼，看起來更像是同步的代碼，請嘗試使用defer.inlineCallbacks

這來自文檔： http : //twisted.readthedocs.io/en/twisted-16.2.0/core/howto/defer-intro.html#inline-callbacks-using-yield

考慮以下以傳統的Deferred樣式編寫的函數：

def getUsers():
   d = makeRequest("GET", "/users")
   d.addCallback(json.loads)
   return d

使用inlineCallbacks，我們可以這樣寫：

from twisted.internet.defer import inlineCallbacks, returnValue

@inlineCallbacks
def getUsers(self):
    responseBody = yield makeRequest("GET", "/users")
    returnValue(json.loads(responseBody))

編輯：

def checkResource(self, resource):
    """ use the message to inquire database
        then set the result to a ip variable
    """
    returned = yield self.units.find({'$and': [{'baseIP':resource},{'status':'free'}]},limit=1,fields={'_id':False,'baseIP':True})
    # replacing callback function
    for ip in returned:
        if ip is not None:
            d = self.updateData(ip,'busy') # if this returns deferred use yield again
            returnValue(d)            
    returnValue("NA")

Answer 2

如果要從延期中獲取實際值，則可以在結果可用后使用。 變得可用需要多長時間取決於它正在等待什么。

看下面的例子。 結果可用之前有任意2秒的延遲，因此嘗試在訪問它之前引發AttributeError

from twisted.internet import defer, reactor


def getDummyData(inputData):
    """
    This function is a dummy which simulates a delayed result and
    returns a Deferred which will fire with that result. Don't try too
    hard to understand this.

    From Twisted docs
    """
    deferred = defer.Deferred()
    # simulate a delayed result by asking the reactor to fire the
    # Deferred in 2 seconds time with the result inputData * 3
    reactor.callLater(2, deferred.callback, inputData * 3)
    return deferred


d = getDummyData(2)

reactor.callLater(1, lambda: print(d))
# Prints: <Deferred at 0x7f4853ed3550>

reactor.callLater(3, lambda: print(d))
# Prints: <Deferred at 0x7f4853ed3550 current result: 6>

reactor.callLater(3, lambda: print(d.result))
# Prints: 6

# Throws AttributeError because Deferred does not have a result after 1 sec
# reactor.callLater(1, lambda: print(d.result))

reactor.callLater(4, reactor.stop)
reactor.run()