Python 如果何避免程序僵尸

import imaplib2, time
from threading import *

# This is the threading object that does all the waiting on 
# the event
class Idler(object):
    def __init__(self, conn):
        self.thread = Thread(target=self.idle)
        self.M = conn
        self.event = Event()

    def start(self):
        self.thread.start()

    def stop(self):
        # This is a neat trick to make thread end. Took me a 
        # while to figure that one out!
        print "evernt is set"
        self.event.set()

    def join(self):
        self.thread.join()

    def test(self):
        return self.thread.is_alive()

    def idle(self):
        # Starting an unending loop here
        while True:
            # This is part of the trick to make the loop stop 
            # when the stop() command is given
            if self.event.isSet():
                return
            self.needsync = False
            # A callback method that gets called when a new 
            # email arrives. Very basic, but that's good.
            def callback(args):
                if not self.event.isSet():
                    self.needsync = True
                    self.event.set()
            # Do the actual idle call. This returns immediately, 
            # since it's asynchronous.
            self.M.idle(callback=callback)
            # This waits until the event is set. The event is 
            # set by the callback, when the server 'answers' 
            # the idle call and the callback function gets 
            # called.
            self.event.wait()
            # Because the function sets the needsync variable,
            # this helps escape the loop without doing 
            # anything if the stop() is called. Kinda neat 
            # solution.
            if self.needsync:
                self.event.clear()
                self.dosync()

    # The method that gets called when a new email arrives. 
    # Replace it with something better.
    def dosync(self):
        print "Got an event!"

# Had to do this stuff in a try-finally, since some testing 
# went a little wrong.....
try:
    # Set the following two lines to your creds and server
    M = imaplib2.IMAP4_SSL("mail.example.com")
    M.login("mylogin","mypassword")
    # We need to get out of the AUTH state, so we just select 
    # the INBOX.
    M.select("INBOX")
    # Start the Idler thread
    idler = Idler(M)
    idler.start()
    # Because this is just an example, exit after 1 minute.
    time.sleep(1*60)
finally:
    # Clean up.
    idler.stop()
    idler.join()
    M.close()
    # This is important!
    M.logout()

class Notifier(Logger):
    def __init__(self, host, user, password, box='INBOX', debug=0, use_ssl=True):
        self.irc = None
        self.idler = None
        self.M = None
        self.messages = {}
        self.cybersquatting = {}
        while True:
            try:
                if use_ssl:
                    self.M = imaplib2.IMAP4_SSL(host, debug=debug)
                else:
                    self.M = imaplib2.IMAP4(host, debug=debug)
                self.M.login(user, password)
                self.M.select(box)
                self.idler = Idler(self.M, self)
                self.idler.start()
                while True:
                    # 这段代码一直执行，用来检查idler这个线程是否在活动
                    # 但有的时候代码在这儿会停止，不明白？
                    # 是不是idler thread还是is_alive?
                    if not self.idler.test():
                        raise ThreadDead()
                    try:
                        self.process_cybersquatting()
                    except:
                        pass
                    time.sleep(2)
            except imaplib2.IMAP4.abort:
                self.error("Disconnected.  Trying again!")
            except Exception as e:
                self.error("imaplib2 except:{}".format(e)
            time.sleep(2)