理解twisted中的reactor和deferred（二）

Deferred可以添加多个回调函数，每个回调函数的结果作为下一个回调函数的参数

代码实例（可在pycharm中运行，摘自 https://twistedmatrix.com/documents/current/core/howto/defer.html）

from twisted.internet import reactor, defer

class Getter:
    def gotResults(self, x):
        """
        The Deferred mechanism provides a mechanism to signal error
        conditions.  In this case, odd numbers are bad.

        This function demonstrates a more complex way of starting
        the callback chain by checking for expected results and
        choosing whether to fire the callback or errback chain
        """
        if self.d is None:
            print("Nowhere to put results")
            return

        d = self.d
        self.d = None
        if x % 2 == 0:
            d.callback(x*3)　　# 执行d的第一个回调函数，也就是_toHTML函数
        else:
            d.errback(ValueError("You used an odd number!")) # 执行d的errorback回调函数，也就是ebPrintError

    def _toHTML(self, r):
        """
        This function converts r to HTML.

        It is added to the callback chain by getDummyData in
        order to demonstrate how a callback passes its own result
        to the next callback
        """
        return "Result: %s" % r　　#返回给第二个回调cbPrintData函数做参数

    def getDummyData(self, x):
        """
        The Deferred mechanism allows for chained callbacks.
        In this example, the output of gotResults is first
        passed through _toHTML on its way to printData.

        Again this function is a dummy, simulating a delayed result
        using callLater, rather than using a real asynchronous
        setup.
        """
        self.d = defer.Deferred()
        # simulate a delayed result by asking the reactor to schedule
        # gotResults in 2 seconds time
        reactor.callLater(2, self.gotResults, x)　　# 2秒后执行gotResults函数
        self.d.addCallback(self._toHTML)　　# 添加第一个回调函数
        return self.d

def cbPrintData(result):
    print(result)

def ebPrintError(failure):
    import sys
    sys.stderr.write(str(failure))

# this series of callbacks and errbacks will print an error message
g = Getter()
d = g.getDummyData(3)
d.addCallback(cbPrintData)
d.addErrback(ebPrintError)

# this series of callbacks and errbacks will print "Result: 12"
g = Getter()
d = g.getDummyData(4)
d.addCallback(cbPrintData) # 添加第二个回调函数
d.addErrback(ebPrintError)

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

　

结果：

官网示例图片：这里的Data Source就是 gotResults 函数，执行Deferred 的callback函数相当于把result给到callback模块，结果一级一级传递给所有的callback函数，任何一级callback出现错误，都将调用errback函数

同时，如果errback没有返回exception 或者 return a twisted.python.failure.Failure instance，将切回callback继续执行