python - Twisted 中的 fatal error 和延迟，停止延迟

Question

我有一个问题，使用 python 中的正常 try except block ，如果出现 fatal error ，则可以返回，例如...

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

那么如何使用 deferreds 来做到这一点，如果我只是返回它就会执行回调，认为错误已得到处理，但我如何通知用户出了问题并破坏了下游链。

====更新===

mg 感谢您的帮助，但它不起作用，即使出现 fatal error ，延迟仍然返回到回调后的内容

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，一个全新的答案可以更好地解释如何 deferreds工作。你应该认为，至少我是这样认为的，程序的流程是一个状态机。成功或失败就像机器的输入一样，可能会改变状态。在您的情况下，您有两种状态:已记录和未记录，以及三个输入:成功登录、身份验证错误以及因服务器问题而无法记录。只有其中一个输入是可恢复的，如果服务器因相同的奇怪问题而无法登录用户，在这种情况下，您可以通过重试登录来恢复问题。这是新代码:

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()

使用三个参数调用代码:最大重试次数，系统应重试用户登录的次数，第三个参数是一个 bool 值，指示用户凭据的正确性。尝试以下调用:0 0 1、3 2 0、3 4 0。

我希望这个例子更具说明性。