IMAP in Python - 3 (Error Handling)

前面的例子無法處理錯誤出現的情形，如果出現了錯誤，程式就會因為沒有呼叫到reactor.stop()造成程式卡住，再來看看twisted中如何處理錯誤呢？由於錯誤發生的時候，控制權可能已經不是當下那個function的了，所以一樣使用callback，而處理錯誤的callback稱之為errback。

直接來看例子：

class IMAPLogic:
    def __init__(self, proto):
        self.proto = proto
        self.factory = proto.factory
        self.logintries = 1
        
        d = self.login()
        d.addCallback(self.loggedin)
        d.addErrback(self.loginerror)
        d.addCallback(self.logout)
        d.addCallback(self.stopreactor)
        
        d.addErrback(self.errorhappen)
        
        print "IMAPLogic.__init__returning."

只要對defered object呼叫addErrback，此errback function就會負責處理當時以上的function，也就是說loggedin()發生的錯誤由loginerror()處理，而logout()與stopreactor()的錯誤也由errorhappen()來處理。其實，如果在login()發生了在loginerror()無法處理的錯誤時，twisted會直接把錯誤丟給下一個errback，此例是loginerror()無法處理的錯誤就傳給errorhappen()>
那就再來看一下那兩個errback function：

def loginerror(self, failure):
        print "Your loging failed (attemp %d times)." % self.logintries
        if self.logintries >= 3:
            print "You have tried to log in three times; I'm giving up."
            return failure
        self.logintries += 1
        
        sys.stdout.write("username:")
        self.factory.username = sys.stdin.readline().strip()
        self.factory.password = getpass.getpass("password:")
        
        d = self.login()
        d.addErrback(self.loginerror)
        return d

在這邊處理的是只有登入超過三次失敗的情形，如果超過三次等情形，就會直接return failure把錯誤傳接下去給errorhappen()。在此function的結尾有增加defered object呼叫自己，如果沒成功就繼續回來此function。

def errorhappen(self, failure):
        print "An error occurred:", failure.getErrorMessage()
        print "Because of the error, I am logging out and stopping reactor..."
        d = self.logout()
        d.addBoth(self.stopreactor)
        return failure

此function是單純把failure的訊息印出來，然後logout()，並且無論logout成功失敗都要呼叫stopreactor()來把reactor.run()中止，避免程式掛著。
以下是程式的兩種output：

cacaegg@cacabook:~/workspace/NetworkProgram/src/IMAP$ t-error.py mx.mgt.ncu.edu.tw cacaegg
Enter password for cacaegg on mx.mgt.ncu.edu.tw:
I have successfully connected to the server!
Logging in...
IMAPLogic.__init__returning.
connectionMade returning
I'm logged in!
Logging out.
Stopping reactor.
cacaegg@cacabook:~/workspace/NetworkProgram/src/IMAP$ t-error.py mx.mgt.ncu.edu.tw cacaegg
Enter password for cacaegg on mx.mgt.ncu.edu.tw:
I have successfully connected to the server!
Logging in...
IMAPLogic.__init__returning.
connectionMade returning
Your loging failed (attemp 1 times).
username:ewfwfewf
password:
Logging in...
Your loging failed (attemp 2 times).
username:efwfew
password:
Logging in...
Your loging failed (attemp 3 times).
You have tried to log in three times; I'm giving up.
An error occurred: Authentication failed.
Because of the error, I am logging out and stopping reactor...
Logging out.
Unhandled error in Deferred:
Traceback (most recent call last):
Failure: twisted.mail.imap4.IMAP4Exception: Authentication failed.
Stopping reactor.

程式連結