举个例子，爬一爬知乎日报的相关数据 http://daily.zhihu.com/
1、获取源码
import requests
url = 'http://daily.zhihu.com/'
res = requests.get(url).text
print(res)
个人喜欢requests，直接访问，发现返回500错误
C:\Python35\python.exe F:/PyCharm/爬虫/daily.py
<html><body><h1>500 Server Error</h1>
An internal server error occured.
</body></html>
Process finished with exit code 0
根据经验判断，是知乎禁止爬虫，需要加上一些伪装，让我们看看加上浏览器伪装效果
import requests
headers = {'User-Agent': 'Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36'}
url = 'http://daily.zhihu.com/'
res = requests.get(url,headers=headers).text
print(res)
看看结果，已经返回我们需要的数据
C:\Python35\python.exe F:/PyCharm/爬虫/daily.py
<!DOCTYPE html><html lang="zh-CN"><head><title>知乎日报 - 每天 3 次，每次 7 分钟</title><meta charset="utf-8"><meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1"><meta name="description" content="在中国，资讯类移动应用的人均阅读时长是 5 分钟，而在知乎日报，这个数字是 21。以独有的方式为你提供最高质、最深度、最有收获的阅读体验。"><link rel="stylesheet" href="/css/base.auto.css"><link rel="stylesheet" href="/css/new_home_v3.auto.css"><script src="/js/jquery.1.9.1.js"></script><script src="/js/new_index_v3/home.js"></script><link rel="shortcut icon" href="/favicon.ico" type="image/x-icon"><base target="_blank"><style>h1,h2,h3 {padding: 0;margin:0}</style><base target="_blank"></head><body class="home"><a href="javascript:;" title="回到顶部" class="back-to-top"></a><div class="header navbar-fixed-top"><div class="container-fixed-width clearfix"><div class="top-nav-link"><a href="javascript:;" data-offset="470"><span>浏览内容</span></a><a href="javascript:;" data-offset="0" class="active"><span>App 下载</span></a></div><h1 class="logo"><a href="http://daily.zhihu.com/" title="知乎日报" class="link-logo">知乎日报</a></h1></div></div><div class="download">
...
但是这种写法是否可以应用到所有的网站，答案是“不”
2、代理设置
有时候同一个IP去爬取同一网站上的内容，久了之后就会被该网站服务器屏蔽。解决方法就是更换IP。这个时候，在对方网站上，显示的不是我们真实地IP地址，而是代理服务器的IP地址。
http://www.xicidaili.com/nn/
西刺代理提供了很多可用的国内IP，可以直接拿来使用。
那么如何在爬虫里加入代理呢，看看requests的官方文档怎么说。http://docs.python-requests.org/zh_CN/latest/user/advanced.html#proxies
如果需要使用代理，你可以通过为任意请求方法提供 proxies 参数来配置单个请求:
import requests
proxies = {
"http": "http://10.10.1.10:3128",
"https": "http://10.10.1.10:1080",
}
requests.get("http://example.org", proxies=proxies)
用法很简单，加入proxies参数即可
import requests
proxies = {
"http": "http://121.201.24.248：8088",
"https": "http://36.249.194.52：8118",
}
headers = {'User-Agent': 'Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36'}
url = 'http://daily.zhihu.com/'
res = requests.get(url,headers=headers，proxies=proxies).text
print(len(res))
为了便于测试，只打印出返回数据的长度
C:\Python35\python.exe F:/PyCharm/爬虫/daily.py
10830
Process finished with exit code 0
发现代理服务器成功爬取知乎日报的信息，内容是10830，故意把代理IP写错一位数，看看结果
import requests
proxies = {
"http": "http://121.201.24.248：8088",
"https": "http://36.249.194.52: 222",
}
headers = {'User-Agent': 'Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36'}
url = 'http://daily.zhihu.com/'
res = requests.get(url,headers=headers,proxies=proxies).text
print(len(res))
我们把"https": "http://36.249.194.52：8118"修改为"https": "http://36.249.194.52: 222"，此时返回的结果如下，发现不能获取网页数据。所以，在使用代理服务器爬去网站时，如果出现异常，要考虑代理IP是否失效了。
当然你也可以写一个爬虫，实时抓取最新的代理IP用来爬取。
Traceback (most recent call last):
File "F:/PyCharm/爬虫/daily.py", line 9, in <module>
res = requests.get(url,headers=headers,proxies=proxies).text
File "C:\Python35\lib\site-packages\requests\api.py", line 70, in get
return request('get', url, params=params, **kwargs)
File "C:\Python35\lib\site-packages\requests\api.py", line 56, in request
return session.request(method=method, url=url, **kwargs)
File "C:\Python35\lib\site-packages\requests\sessions.py", line 488, in request
resp = self.send(prep, **send_kwargs)
File "C:\Python35\lib\site-packages\requests\sessions.py", line 609, in send
r = adapter.send(request, **kwargs)
File "C:\Python35\lib\site-packages\requests\adapters.py", line 485, in send
raise ProxyError(e, request=request)
requests.exceptions.ProxyError: HTTPConnectionPool(host='121.201.24.248：8088', port=80): Max retries exceeded with url: http://daily.zhihu.com/ (Caused by ProxyError('Cannot connect to proxy.', NewConnectionError('<requests.packages.urllib3.connection.HTTPConnection object at 0x0000000003860DA0>: Failed to establish a new connection: [Errno 11004] getaddrinfo failed',)))
3、模拟登录
有些网站是需要登录才能看到信息的，例如知乎，直接用requests获取知乎首页信息，返回数据是需要你登录的，只有登录了才能看到数据。
<button type="button" class="signin-switch-button">手机验证码登录</button>
<a class="unable-login" href="#">无法登录？</a>
</div>
<div class="social-signup-wrapper" data-za-module="SNSSignIn">
<span class="name js-toggle-sns-buttons">社交帐号登录</span>
<div class="sns-buttons">
<a title="微信登录" class="js-bindwechat" href="#"><i class="sprite-index-icon-wechat"></i></a>
<a title="微博登录" class="js-bindweibo" href="#"><i class="sprite-index-icon-weibo"></i></a>
<a title="QQ 登录" class="js-bindqq" href="#"><i class="sprite-index-icon-qq"></i></a>
</div>
再次回到官方文档http://docs.python-requests.org/zh_CN/latest/user/quickstart.html#cookie
如果某个响应中包含一些 cookie，你可以快速访问它们：
>>> url = 'http://example.com/some/cookie/setting/url'
>>> r = requests.get(url)
>>> r.cookies['example_cookie_name']
'example_cookie_value'
要想发送你的cookies到服务器，可以使用 cookies 参数：
>>> url = 'http://httpbin.org/cookies'
>>> cookies = dict(cookies_are='working')
>>> r = requests.get(url, cookies=cookies)
>>> r.text
'{"cookies": {"cookies_are": "working"}}'
具体的分析过程可以参考xchaoinfo所写的文章和视频，讲解十分清晰
https://zhuanlan.zhihu.com/p/25633789
下面是代码
import requests
from bs4 import BeautifulSoup
import os, time
import re
# import http.cookiejar as cookielib
# 构造 Request headers
agent = 'Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Mobile Safari/537.36'
headers = {
"Host": "www.zhihu.com",
"Referer": "https://www.zhihu.com/",
'User-Agent': agent
}
######### 构造用于网络请求的session
session = requests.Session()
# session.cookies = cookielib.LWPCookieJar(filename='zhihucookie')
# try:
# session.cookies.load(ignore_discard=True)
# except:
# print('cookie 文件未能加载')
############ 获取xsrf_token
homeurl = 'https://www.zhihu.com'
homeresponse = session.get(url=homeurl, headers=headers)
homesoup = BeautifulSoup(homeresponse.text, 'html.parser')
xsrfinput = homesoup.find('input', {'name': '_xsrf'})
xsrf_token = xsrfinput['value']
print("获取到的xsrf_token为： ", xsrf_token)
########## 获取验证码文件
randomtime = str(int(time.time() * 1000))
captchaurl = 'https://www.zhihu.com/captcha.gif?r='+\
randomtime+"&type=login"
captcharesponse = session.get(url=captchaurl, headers=headers)
with open('checkcode.gif', 'wb') as f:
f.write(captcharesponse.content)
f.close()
# os.startfile('checkcode.gif')
captcha = input('请输入验证码：')
print(captcha)
########### 开始登陆
headers['X-Xsrftoken'] = xsrf_token
headers['X-Requested-With'] = 'XMLHttpRequest'
loginurl = 'https://www.zhihu.com/login/email'
postdata = {
'_xsrf': xsrf_token,
'email': '邮箱@qq.com',
'password': '密码'
}
loginresponse = session.post(url=loginurl, headers=headers, data=postdata)
print('服务器端返回响应码：', loginresponse.status_code)
print(loginresponse.json())
# 验证码问题输入导致失败: 猜测这个问题是由于session中对于验证码的请求过期导致
if loginresponse.json()['r']==1:
# 重新输入验证码，再次运行代码则正常。也就是说可以再第一次不输入验证码，或者输入一个错误的验证码，只有第二次才是有效的
randomtime = str(int(time.time() * 1000))
captchaurl = 'https://www.zhihu.com/captcha.gif?r=' + \
randomtime + "&type=login"
captcharesponse = session.get(url=captchaurl, headers=headers)
with open('checkcode.gif', 'wb') as f:
f.write(captcharesponse.content)
f.close()
os.startfile('checkcode.gif')
captcha = input('请输入验证码：')
print(captcha)
postdata['captcha'] = captcha
loginresponse = session.post(url=loginurl, headers=headers, data=postdata)
print('服务器端返回响应码：', loginresponse.status_code)
print(loginresponse.json())
##########################保存登陆后的cookie信息
# session.cookies.save()
############################判断是否登录成功
profileurl = 'https://www.zhihu.com/settings/profile'
profileresponse = session.get(url=profileurl, headers=headers)
print('profile页面响应码：', profileresponse.status_code)
profilesoup = BeautifulSoup(profileresponse.text, 'html.parser')
div = profilesoup.find('div', {'id': 'rename-section'})
print(div)
好了关于爬虫的第一步，获取源码这一节讲了很多，其实大多数网站加上User-Agent和代理IP就可以正常爬取。下一节会讲讲如何利用xpath来解析网页，获取我们想要的数据。
喜欢爬虫、数据的可以关注一下我的微信公众号，多多交流。
