如何从返回的延迟实例中获取值

Question

我使用 txmongo 库作为 mongoDB 的驱动程序。在其有限的文档中，txmongo 中的 find 函数将 return 一个 deferred 实例，但我怎样才能得到实际结果（如 {"IP":11.12.59.119}）？我尝试了 yield、str() 和 repr() 但没有用。

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

如果你想在 Twisted 中编写看起来更像同步的异步代码，请尝试使用 defer.inlineCallbacks

这是来自文档： http://twisted.readthedocs.io/en/twisted-16.2.0/core/howto/defer-intro.html#inline-callbacks-using-yield

Consider the following function written in the traditional Deferred style:

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

using inlineCallbacks, we can write this as:

from twisted.internet.defer import inlineCallbacks, returnValue

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

编辑：

from twisted.internet.defer import inlineCallbacks, returnValue

@inlineCallbacks
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

如果您想从 deferred 中获取实际值，可以在结果可用后进行。需要多长时间才能可用取决于它在等待什么。

看看下面的例子。在结果可用之前有任意 2 秒的延迟，因此在此之前尝试访问它会抛出 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()

如何从返回的延迟实例中获取值

how to get value from returned instance of deferred

python

twisted

mongodb