python爬虫 设置了去重但是还是爬几条后就无限爬同一个URL，下面是去重的代码

Question

class UrlManager(object):    def __init__(self):        self.new_urls = set()        self.old_urls = set()    def add_new_url(self, url):        if url is None:           return        if url not in self.new_urls and url not in self.old_urls:            self.new_urls.add(url)    def add_new_urls(self, urls):        if urls is None or len(urls) == 0:            return        for url in urls:            self.new_urls.add(url)    def has_new_url(self):        return len(self.new_urls) != 0    def get_new_url(self):        new_url = self.new_urls.pop()        self.old_urls.add(new_url)        return new_url