Twisted中的致命错误与Deferred，如何停止Deferred

Question

我遇到了一个问题。在Python中，正常的try except块可以在出现严重错误时直接返回，比如...

try:
    logon()
except 404_Error:
    retry_logon(try = 2)

except AuthenticationProblem:
    error_label.SetText( "You password or username was wrong. Make sure that CAPS LOCK key is not on." )
    return#nothing more we can do here
else:
    #display user information or whatever

那么在使用deferreds的时候该怎么做呢？如果我直接返回，程序会继续执行回调，认为错误已经处理了。但我该如何通知用户出了问题，并且终止后续的操作呢？

==== 更新 ===

谢谢你的帮助，但这方法不管用，即使出现了严重错误，defer仍然会在之后返回到回调。

from twisted.internet import reactor
from twisted.internet.defer import Deferred as D

class NonFatalError(Exception):
    'non fatal error'
class FatalError(Exception):
    'fatal error'
def c(s):
    print "Callback called"
    print "Data Received: %s" % s
def e(f):
    print "Errorback Called"
    print "Error Type: %s" % type(f)
    print "Traceback"
    f.printTraceback()
    print "======================================="
    f.trap(NonFatalError)
    return "Error Handled"
def e_fatal(f, d):
    print "Errorback Called"
    print "Error Type: %s" % type(f)
    print "Traceback"
    f.printTraceback()
    print "======================================="
    print "Fatal Error"
    f.trap(FatalError)
    return "Fatal Error... Crash and die. No more callbacks should be called."

def trigger():
    d.errback(FatalError("This error is fatal to the defer"))

if __name__ == "__main__":
    d = D()
    d.addErrback(e)
    d.addErrback(e_fatal, d)
    d.addCallback(c)
    d.addCallback(c)
    d.addCallback(c)
    d.addCallback(c)
    reactor.callLater(3, trigger)
    reactor.callLater(10, reactor.stop)
    reactor.run()

    raw_input("Done.")

Answer 1

好的，我来给你解释一下 deferreds 是怎么工作的。你可以把程序的流程想象成一个状态机。成功或者失败就像是这个机器的输入，可能会改变它的状态。在你的例子中，有两个状态：已登录和未登录，还有三个输入：成功登录、认证错误和因为服务器问题无法登录。只有一个输入是可以恢复的，那就是服务器因为某些奇怪的问题无法登录用户，在这种情况下，你可以通过重试登录来解决问题。下面是新的代码：

import sys
from twisted.internet import reactor, defer


class FourOhFourError(Exception):
    pass


class AuthenticationError(Exception):
    pass


def logon(retry=3, success=2, wrong_auth=0):
    # do stuff
    d = defer.Deferred()
    # not_found is the only error recoverable
    d.addErrback(not_found, retry, success)
    if wrong_auth:
        reactor.callLater(0, d.errback, AuthenticationError("wrong auth"))
    else:
        if success == 0:
            reactor.callLater(0, d.callback, "Mario")
        else:
            reactor.callLater(0, d.errback, FourOhFourError("Not found"))
    return d


def not_found(failure, retry, success):
    failure.trap(FourOhFourError) # this is superfluous here
    print failure.getErrorMessage()
    if retry == 0:
        raise AuthenticationError("Max retries")
    # do stuff
    print "retring..."
    d = defer.Deferred()
    d.addCallback(logon, success-1)
    reactor.callLater(1, d.callback, retry-1) # not really clean here
    return d


def wrong_auth(failure):
    failure.trap(AuthenticationError) # this is superfluous here
    # do stuff
    print "something goes wrong"
    print failure.getErrorMessage()


def loggedIn(user):
    print "hello %s" % user


def stop(_):
    reactor.stop()


d = logon(*map(int, sys.argv[1:]))
d.addCallbacks(loggedIn, wrong_auth)
d.addBoth(stop)
reactor.run()

调用这个代码时需要三个参数：最大重试次数、在第几次重试时系统应该登录用户，第三个是一个布尔值，表示用户凭证是否正确。你可以尝试以下调用： 0 0 1、3 2 0、3 4 0。

希望这个例子能更清楚地说明问题。