我在python写了下面的程序很简单的网络爬虫,但是当我运行它,它返回我"NoneType"对象不是可调用的",你能帮帮我吗？
import BeautifulSoup
import urllib2
def union(p,q):
for e in q:
if e not in p:
p.append(e)
def crawler(SeedUrl):
tocrawl=[SeedUrl]
crawled=[]
while tocrawl:
page=tocrawl.pop()
pagesource=urllib2.urlopen(page)
s=pagesource.read()
soup=BeautifulSoup.BeautifulSoup(s)
links=soup('a')
if page not in crawled:
union(tocrawl,links)
crawled.append(page)
return crawled
crawler('http://www.princeton.edu/main/')