不会写重试，我用了这么个方法来 作为重试机制：
    def web(self):
        time.sleep(5)
        i = 0
        a = self.br.current_url
        print a
        while i < 10:
            self.br.back()
            if self.br.current_url == a:
                i+=1
            else:
                i=11