请教一道 Python 多线程爬虫的面试题

start = "http://google.com"
queue = [start]
visited = {start}

while queue:
    url = queue.pop(0)
    print url

    for next_url in extract_urls(url):
        if next_url not in visited:
            queue.append(next_url)
            visited.add(next_url)

class ThreadUrl(threading.Thread):

    def __init__(self, queue, visited):
        super(ThreadUrl, self).__init__()
        self.queue = queue
        self.visited = visited

    def run(self):
        while True:
            url = self.queue.get()
            print "%s: %s" % (self.name, url)
            for next_url in extract_urls(url):
                if next_url not in self.visited:
                    self.queue.put(next_url)
                    self.visited.add(next_url)
            self.queue.task_done()


queue = Queue()
visited = set()
visited.add("http://google.com")

for i in range(5):
    t = ThreadUrl(queue, visited)
    t.setDaemon(True)
    t.start()

queue.put(start_url)
queue.join()

class ThreadUrl(threading.Thread):

    def __init__(self, queue, visited, lock):
        super(ThreadUrl, self).__init__()
        self.queue = queue
        self.visited = visited
        self.lock = lock

    def run(self):
        while True:
            url = self.queue.get()
            print "%s: %s" % (self.name, url)
            for next_url in extract_urls(url):
                self.lock.acquire()
                if next_url not in self.visited:
                    self.queue.put(next_url)
                    self.visited.add(next_url)
                self.lock.release()
            self.queue.task_done()