这个报错是什么原因？
Traceback (most recent call last):
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/spider_main.py", line 48, in <module>
obj_spider.craw(root_url)
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/spider_main.py", line 21, in craw
self.urls.add_new_url(root_url)
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/url_manager.py", line 19, in add_new_url
self.add_new_urls(url)
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/url_manager.py", line 25, in add_new_urls
self.add_new_url(url)
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/url_manager.py", line 19, in add_new_url
self.add_new_urls(url)
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/url_manager.py", line 25, in add_new_urls
self.add_new_url(url)
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/url_manager.py", line 19, in add_new_url
self.add_new_urls(url)
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/url_manager.py", line 25, in add_new_urls
self.add_new_url(url)
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/url_manager.py", line 19, in add_new_url
self.add_new_urls(url)
File "/Users/lixuan/Documents/workspace/imooc/baike_spider/url_manager.py", line 25, in add_new_urls
self.add_new_url(url)
self.add_new_url(url)
一下省略数行
RuntimeError: maximum recursion depth exceeded
