urllib提供了一系列用于操作URL的功能。
Get
urllib的request模块可以非常方便地抓取URL内容，也就是发送一个GET请求到指定的页面，然后返回HTTP的响应：
例如，对豆瓣的一个URLhttps://api.douban.com/v2/book/2129650进行抓取，并返回响应：
from urllib import request
with request.urlopen("https://api.douban.com/v2/book/2129650") as f:
data = f.read()
print("Status:", f.status, f.reason)
for k, v in f.getheaders():
print("%s: %s" % (k, v))
print("Data:", data.decode("utf-8"))
可以看到HTTP响应的头和JSON数据：
Status: 200 OK
Server: nginx
Date: Tue, 26 May 2015 10:02:27 GMT
Content-Type: application/json; charset=utf-8
Content-Length: 2049
Connection: close
Expires: Sun, 1 Jan 2006 01:00:00 GMT
Pragma: no-cache
Cache-Control: must-revalidate, no-cache, private
X-DAE-Node: pidl1
Data: {"rating":{"max":10,"numRaters":16,"average":"7.4","min":0},"subtitle":"","author":["廖雪峰编著"],"pubdate":"2007-6","tags":[{"count":20,"name":"spring","title":"spring"}...}
如果我们要想模拟浏览器发送GET请求，就需要使用Request对象，通过往Request对象添加HTTP头，我们就可以把请求伪装成浏览器。例如，模拟iPhone 6去请求豆瓣首页：
from urllib import request
req = request.Request("http://www.douban.com/")
req.add_header("User-Agent", "Mozilla/6.0 (iPhone; CPU iPhone OS 8_0 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/8.0 Mobile/10A5376e Safari/8536.25")
with request.urlopen(req) as f:
print("Status:", f.status, f.reason)
for k, v in f.getheaders():
print("%s: %s" % (k, v))
print("Data:", f.read().decode("utf-8"))
这样豆瓣会返回适合iPhone的移动版网页：
...
<meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0">
<meta name="format-detection" content="telephone=no">
<link rel="apple-touch-icon" sizes="57x57" href="http://img4.douban.com/pics/cardkit/launcher/57.png" />
...
Post
如果要以POST发送一个请求，只需要把参数data以bytes形式传入。
我们模拟一个微博登录，先读取登录的邮箱和口令，然后按照weibo.cn的登录页的格式以username=xxx&password=xxx的编码传入：
from urllib import request, parse
print("Login to weibo.cn...")
email = input("Email: ")
passwd = input("Password: ")
login_data = parse.urlencode([
("username", email),
("password", passwd),
("entry", "mweibo"),
("client_id", ""),
("savestate", "1"),
("ec", ""),
("pagerefer", "https://passport.weibo.cn/signin/welcome?entry=mweibo&r=http%3A%2F%2Fm.weibo.cn%2F")
])
req = request.Request("https://passport.weibo.cn/sso/login")
req.add_header("Origin", "https://passport.weibo.cn")
req.add_header("User-Agent", "Mozilla/6.0 (iPhone; CPU iPhone OS 8_0 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/8.0 Mobile/10A5376e Safari/8536.25")
req.add_header("Referer", "https://passport.weibo.cn/signin/login?entry=mweibo&res=wel&wm=3349&r=http%3A%2F%2Fm.weibo.cn%2F")
with request.urlopen(req, data=login_data.encode("utf-8")) as f:
print("Status:", f.status, f.reason)
for k, v in f.getheaders():
print("%s: %s" % (k, v))
print("Data:", f.read().decode("utf-8"))
如果登录成功，我们获得的响应如下：
Status: 200 OK
Server: nginx/1.2.0
...
Set-Cookie: SSOLoginState=1432620126; path=/; domain=weibo.cn
...
Data: {"retcode":20000000,"msg":"","data":{...,"uid":"1658384301"}}
如果登录失败，我们获得的响应如下：
...
Data: {"retcode":50011015,"msg":"u7528u6237u540du6216u5bc6u7801u9519u8bef","data":{"username":"example@python.org","errline":536}}
Handler
如果还需要更复杂的控制，比如通过一个Proxy去访问网站，我们需要利用ProxyHandler来处理，示例代码如下：
proxy_handler = urllib.request.ProxyHandler({"http": "http://www.example.com:3128/"})
proxy_auth_handler = urllib.request.ProxyBasicAuthHandler()
proxy_auth_handler.add_password("realm", "host", "username", "password")
opener = urllib.request.build_opener(proxy_handler, proxy_auth_handler)
with opener.open("http://www.example.com/login.html") as f:
pass
小结
urllib提供的功能就是利用程序去执行各种HTTP请求。如果要模拟浏览器完成特定功能，需要把请求伪装成浏览器。伪装的方法是先监控浏览器发出的请求，再根据浏览器的请求头来伪装，User-Agent头就是用来标识浏览器的。
练习
利用urllib读取XML，将XML一节的数据由硬编码改为由urllib获取：
from urllib import request, parse
def fetch_xml(url):
# 测试
print(fetch_xml("http://weather.yahooapis.com/forecastrss?u=c&w=2151330"))
Run