提示object has no attribute 'new_urls'，看了好多遍，和老师写的一样啊，也没空格大小写问题
Traceback (most recent call last):
File "C:\eclipse\workspace\baiduspider\spider_main.py", line 50, in <module>
obj_spider.craw(root_url)
File "C:\eclipse\workspace\baiduspider\spider_main.py", line 19, in craw
self.urls.add_new_url(root_url)
File "C:\eclipse\workspace\baiduspider\url_manager.py", line 11, in add_new_url
if url not in self.new_urls and url not in self.old_urls:
AttributeError: 'UrlManager' object has no attribute 'new_urls'
