详解Python爬虫的基本写法
什么是爬虫
爬虫，即网络爬虫，大家可以理解为在网络上爬行的一直蜘蛛，互联网就比作一张大网，而爬虫便是在这张网上爬来爬去的蜘蛛咯，如果它遇到资源，那么它就会抓取下来。想抓取什么？这个由你来控制它咯。
比如它在抓取一个网页，在这个网中他发现了一条道路，其实就是指向网页的超链接，那么它就可以爬到另一张网上来获取数据。这样，整个连在一起的大网对这之蜘蛛来说触手可及，分分钟爬下来不是事儿。
1.最基本的抓站
import urllib2
content = urllib2.urlopen('http://XXXX').read()2.使用代理服务器
这在某些情况下比较有用，比如IP被封了，或者比如IP访问的次数受到限制等等。
import urllib2
proxy_support = urllib2.ProxyHandler({'http':'http://XX.XX.XX.XX:XXXX'})
opener = urllib2.build_opener(proxy_support, urllib2.HTTPHandler)
urllib2.install_opener(opener)
content = urllib2.urlopen('http://XXXX').read()3.需要登录的情况
登录的情况比较麻烦我把问题拆分一下：
3.1.cookie的处理
import urllib2, cookielib
cookie_support= urllib2.HTTPCookieProcessor(cookielib.CookieJar())
opener = urllib2.build_opener(cookie_support, urllib2.HTTPHandler)
urllib2.install_opener(opener)
content = urllib2.urlopen('http://XXXX').read()是的没错，如果想同时用代理和cookie，那就加入proxy_support然后operner改为
opener = urllib2.build_opener(proxy_support, cookie_support, urllib2.HTTPHandler)
3.2 表单的处理
登录必要填表，表单怎么填？首先利用工具截取所要填表的内容
比如我一般用firefox+httpfox插件来看看自己到底发送了些什么包
这个我就举个例子好了，以verycd为例，先找到自己发的POST请求，以及POST表单项：
可以看到verycd的话需要填username,password,continueURI,fk,login_submit这几项，其中fk是随机生成的（其实不太随机，看上去像是把epoch时间经过简单的编码生成的），需要从网页获取，也就是说得先访问一次网页，用正则表达式等工具截取返回数据中的fk项。continueURI顾名思义可以随便写，login_submit是固定的，这从源码可以看出。还有username，password那就很显然了。
好的，有了要填写的数据，我们就要生成postdata
import urllib
postdata=urllib.urlencode({
'username':'XXXXX',
'password':'XXXXX',
'continueURI':'http://www.verycd.com/',
'fk':fk,
'login_submit':'登录'
})然后生成http请求，再发送请求：
req = urllib2.Request( url = 'http://secure.verycd.com/signin/*/http://www.verycd.com/', data = postdata ) result = urllib2.urlopen(req).read()
3.3 伪装成浏览器访问
某些网站反感爬虫的到访，于是对爬虫一律拒绝请求
这时候我们需要伪装成浏览器，这可以通过修改http包中的header来实现
#…
headers = {
'User-Agent':'Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.6) Gecko/20091201 Firefox/3.5.6'
}
req = urllib2.Request(
url = 'http://secure.verycd.com/signin/*/http://www.verycd.com/',
data = postdata,
headers = headers
)
#...3.4 反”反盗链”
某些站点有所谓的反盗链设置，其实说穿了很简单，就是检查你发送请求的header里面，referer站点是不是他自己，所以我们只需要像3.3一样，把headers的referer改成该网站即可，以黑幕著称地cnbeta为例：
#...
headers = {
'Referer':'http://www.cnbeta.com/articles'
}
#...headers是一个dict数据结构，你可以放入任何想要的header，来做一些伪装。例如，有些自作聪明的网站总喜欢窥人隐私，别人通过代理访问，他偏偏要读取header中的X-Forwarded-For来看看人家的真实IP，没话说，那就直接把X-Forwarde-For改了吧，可以改成随便什么好玩的东东来欺负欺负他，呵呵。
3.5 终极绝招
有时候即使做了3.1-3.4，访问还是会被据，那么没办法，老老实实把httpfox中看到的headers全都写上，那一般也就行了。
再不行，那就只能用终极绝招了，selenium直接控制浏览器来进行访问，只要浏览器可以做到的，那么它也可以做到。类似的还有pamie，watir，等等等等。
4.多线程并发抓取
print time.time()-start
1..
from threading import Thread
from Queue import Queue
from time import sleep
import urllib2
import time
q = Queue()
NUM = 10
JOBS = 50
def do_somthing_using(p):
response = urllib2.urlopen('http://www.cnblogs.com')
result = response.read()
#print p
def working():
while True:
arguments = q.get()
do_somthing_using(arguments)
q.task_done()
for i in xrange(NUM):
t = Thread(target=working)
t.setDaemon(True)
t.start()
start = time.time()
for i in xrange(JOBS):
q.put(i)
q.join()
print "MultiThreading:"
print time.time()-start
2..
from threading import Thread
from multiprocessing.dummy import Pool as ThreadPool
import urllib2
import time
start = time.time()
url = "http://www.cnblogs.com"
urls = [url] * 50
pool = ThreadPool(4)
results = pool.map(urllib2.urlopen, urls)
pool.close()
pool.join()
print "Map:"
print time.time()-start关于Python爬虫的基本写法小编就给大家介绍到这里，后续还会持续更细，敬请关注，谢谢！
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