python/twisted - 这段代码有什么问题？

Question

我正在学习 Python，目前专注于 Twisted...我正在做一个教程练习，但我似乎无法理解为什么第二版代码中的修改会导致“计数”功能在反应堆启动之前执行。所改变的只是将 's' 参数添加到函数中。

工作：

class Countdown(object):

counter = 5

def count(self):
    if self.counter == 0:
        reactor.stop()
    else:
        print self.counter, '...'
        self.counter -= 1
        reactor.callLater(1, self.count)

from twisted.internet import reactor

reactor.callWhenRunning(Countdown().count)

print 'Start!'
reactor.run()
print 'Stop!'

破碎：

class Countdown(object):

counter = 5

def count(self,s):
    if self.counter == 0:
        reactor.stop()
    else:
        print self.counter, '...'
        self.counter -= 1
        reactor.callLater(1, self.count(1))

from twisted.internet import reactor

reactor.callWhenRunning(Countdown().count(1))

print 'Start!'
reactor.run()
print 'Stop!'

这是回溯：

Traceback (most recent call last):
  File "C:\Python27\twistedexample\basic-twisted\countdown.py", line 15, in <module>
    reactor.callWhenRunning(Countdown().count(1))
  File "C:\Python27\twistedexample\basic-twisted\countdown.py", line 11, in count
    reactor.callLater(1, self.count(1))
  File "C:\Python27\twistedexample\basic-twisted\countdown.py", line 11, in count
    reactor.callLater(1, self.count(1))
  File "C:\Python27\twistedexample\basic-twisted\countdown.py", line 11, in count
    reactor.callLater(1, self.count(1))
  File "C:\Python27\twistedexample\basic-twisted\countdown.py", line 11, in count
    reactor.callLater(1, self.count(1))
  File "C:\Python27\twistedexample\basic-twisted\countdown.py", line 11, in count
    reactor.callLater(1, self.count(1))
  File "C:\Python27\twistedexample\basic-twisted\countdown.py", line 7, in count
    reactor.stop()
  File "C:\Python27\lib\site-packages\twisted\internet\base.py", line 580, in stop
  "Can't stop reactor that isn't running.")
  ReactorNotRunning: Can't stop reactor that isn't running.

对此的任何意见表示赞赏，我觉得我在这里遗漏了一些重要的东西，不想跳过它。

Answer 1

代码的第一个版本传递了对函数 count 的引用（以便以后可以调用它），而损坏的代码版本传递了函数的 result count(1) 的函数调用，它将是 None（因为 count 没有 return 值）。所以你所做的基本上是改变了这个：

reactor.callWhenRunning(Countdown().count)

到这里：

reactor.callWhenRunning(None)

除此之外，您立即调用count(1)，而不是注册它以供以后调用！这解释了您看到的错误，因为在您到达reactor.run() 行之前，倒计时正在运行。

这同样适用于调用count(1) 的行reactor.callLater(1, self.count(1))，可能返回None，实际上并没有向callLater 注册任何函数。