使用请求和美丽的汤登录到asp网站

Question

我正在尝试编写一个将登录到asp网站的程序。每次我运行程序时它都会给我这个错误代码：

Traceback (most recent call last):
  File "/home/icecreamlolcat/python/2.py", line 17, in <module>
    EVENTVALIDATION=soup.find(id="__EVENTVALIDATION")['value']
TypeError: 'NoneType' object has no attribute '__getitem__'

这是我的代码：

import requests
from bs4 import BeautifulSoup

URL="website"
headers={"User-Agent":"Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2062.120 Safari/537.36"}

username="username"
password="password"

s=requests.Session()
s.headers.update(headers)
r=s.get(URL)
soup=BeautifulSoup(r.content)

VIEWSTATE=soup.find(id="__VIEWSTATE")['value']
VIEWSTATEGENERATOR=soup.find(id="__VIEWSTATEGENERATOR")['value']
EVENTVALIDATION=soup.find(id="__EVENTVALIDATION")['value']

login_data={"__VIEWSTATE":VIEWSTATE,
"__VIEWSTATEGENERATOR":VIEWSTATEGENERATOR,
"__EVENTVALIDATION":EVENTVALIDATION,
"ctl00$ContentPlaceHolderNavPane$LeftSection$UserLogin$UserName":username,
"ctl00$ContentPlaceHolderNavPane$LeftSection$UserLogin$Password":password,
"ctl00$ContentPlaceHolderNavPane$LeftSection$UserLogin$LoginButton":"Log In"}

r=s.post(URL, data=login_data)
print r.url