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使用Python的urllib和urllib2模块制作爬虫的实例教程

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Jun 10, 2016 pm 03:06 PM

python urllib reptile

urllib
学习python完基础,有些迷茫.眼睛一闭,一种空白的窒息源源不断而来.还是缺少练习,遂拿爬虫来练练手.学习完斯巴达python爬虫课程后,将心得整理如下,供后续翻看.整篇笔记主要分以下几个部分:

1.做一个简单的爬虫程序
2.小试牛刀--抓取百度贴吧图片
3.总结

1.做一个简单的爬虫程序
首先环境描述

Device: Mba 2012 Yosemite 10.10.1
Python: python 2.7.9
编辑器: Sublime Text 3

这个没有什么好说的,直接上代码吧!

'''
@ urllib为python自带的一个网络库
@ urlopen为urllib的一个方法,用于打开一个连接并抓取网页,
 然后通过read()方法把值赋给read()
'''
import urllib
 
url = "http://www.lifevc.com"#多嘴两句,为什么要选lifevc呢,主要是最近它很惹我.
html = urllib.urlopen(url)
content = html.read()
html.close()
#可以通过print打印出网页内容
print content

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很简单,基本上没有可说的,这个也就是python的魅力,几行代码就完成.
当然我们仅仅抓取网页,没有实在的价值.接下来我们就开始做一点有意义的事情.

2.小试牛刀
抓取百度贴吧图片
其实也很简单,因为要抓取图片,还需要先分析一下网页源代码
(这里以知道基本html知识,浏览器以chrome为例)
如图,这里简要说下步骤,请参考.

打开网页,右键点击,选择"inspect Element"(最下面这一项)
点击下面弹起来的框框最左边那个问号,问号会变成蓝色
移动鼠标去点击我们想要抓取的图片(一个萌妹子)
如图,我们就可以图片在源码中的位置了

2016120102212622.gif (899×570)

下面将源码相关拷贝出来

1 2	`<img` `class="BDE_Image lazy"` `src="/static/imghw/default1.png"` `data-src="(.+?\.jpg)"` `style="max-width:90%"` `style="max-width:90%"` `style="cursor: url(http://tb2.bdstatic.com/tb/` `static-pb/img/cur_zin.cur), pointer;">`

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经分析和对比(这里略掉),基本上可以看到要抓取的图片几个特征:

在img标签下
在名为BDE_Image的类下面
图片格式为jpg

正则表达式后续我会更新,请关注

依照上述判断,直接上代码

'''
@本程序用来下载百度贴吧图片
@re 为正则说明库
'''
import urllib
import re
 
# 获取网页html信息
url = "http://tieba.baidu.com/p/2336739808"
html = urllib.urlopen(url)
content = html.read()
html.close()
 
# 通过正则匹配图片特征,并获取图片链接
img_tag = re.compile(r'')
img_links = re.findall(img_tag, content)
 
# 下载图片 img_counter为图片计数器(文件名)
img_counter = 0
for img_link in img_links:
  img_name = '%s.jpg' % img_counter
  urllib.urlretrieve(img_link, "//Users//Sean//Downloads//tieba//%s" %img_name)
  img_counter += 1

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如图,我们就抓取你懂的图片

2016120102253240.png (908×659)

3.总结
如上两节,我们就很轻松的就可以网页或者图片.
补充一点小技巧,如果遇到不是很明白的库或者方法,可以通过以下方法进行初步了解.

dir(urllib) #查看当前库有哪些方法
help(urllib.urlretrieve) #查看跟当前方法相关的作用或者参数,官方比较权威

或者https://docs.python.org/2/library/index.html进项相关搜索.

当然百度也可以,但是效率太低.建议使用 http://xie.lu 进行相关搜索(你懂了,绝对满意).
这里我们讲解如何抓取网页和下载图片,在下面我们会讲解如何抓取有限制抓取的网站.

urllib2
上面我们讲解如何抓取网页和下载图片,在下一节里面我们会讲解如何抓取有限制抓取的网站
首先,我们依然用我们上一节课的方法去抓取一个大家都用来举例的网站,本文主要分以下几个部分:

1.抓取受限网页
2.对代码进行一些优化

1.抓取受限网页

首先使用我们上一节学到的知识测试一下:

'''
@本程序用来抓取blog.csdn.net网页
'''
import urllib
 
url = "http://blog.csdn.net/FansUnion"
html = urllib.urlopen(url)
#getcode()方法为返回Http状态码
print html.getcode()
html.close()
#输出

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403

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此处我们的输出为403,代表拒绝访问;同理200表示请求成功完成;404表示网址未找到.
可见csdn已做了相关屏蔽,通过第一节的方法是无法获取网页,在这里我们需要启动一个新的库:urllib2
但是我们也看到浏览器可以发那个文,是不是我们模拟浏览器操作,就可以获取网页信息.
老办法,我们先来看看浏览器是如何提交请求给csdn服务器的.首先简述一下方法:

打开网页,右键点击,选择"inspect Element"(最下面这一项)
点击下面弹起来的框框的Network选项卡
刷新网页,就可以看到Network选项卡抓取了很多信息
找到其中一个信息展开,就能看到请求包的Header

2016120102427056.gif (856×706)

以下就是整理后的Header信息

Request Method:GET
Host:blog.csdn.net
Referer:http://blog.csdn.net/&#63;ref=toolbar_logo
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/38.0.2125.104 Safari/537.36

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然后根据提取的Header信息,利用urllib2的Request方法模拟浏览器向服务器提交请求,代码如下:

# coding=utf-8
'''
@本程序用来抓取受限网页(blog.csdn.net)
@User-Agent:客户端浏览器版本
@Host:服务器地址
@Referer:跳转地址
@GET:请求方法为GET
'''
import urllib2
 
url = "http://blog.csdn.net/FansUnion"
 
#定制自定义Header,模拟浏览器向服务器提交请求
req = urllib2.Request(url)
req.add_header('User-Agent', 'Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.95 Safari/537.36')
req.add_header('Host', 'blog.csdn.net')
req.add_header('Referer', 'http://blog.csdn.net')
req.add_header('GET', url)
 
#下载网页html并打印
html = urllib2.urlopen(req)
content = html.read()
print content
html.close()

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呵呵,你限制我,我就跳过你的限制.据说只要浏览器能够访问的,就能够通过爬虫抓取.

2.对代码进行一些优化
简化提交Header方法
发现每次写那么多req.add_header对自己来说是一种折磨,有没有什么方法可以只要复制过来就使用.答案是肯定的.

#input:
help(urllib2.Request)
#output(因篇幅关系,只取__init__方法)
__init__(self, url, data=None, headers={}, origin_req_host=None, unverifiable=False)
通过观察,我们发现headers={},就是说可以以字典的方式提交header信息.那就动手试试咯!!
 
#只取自定义Header部分代码
csdn_headers = {
  "User-Agent": "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.95 Safari/537.36",
  "Host": "blog.csdn.net",
  'Referer': 'http://blog.csdn.net',
  "GET": url
  }
req = urllib2.Request(url,headers=csdn_headers)

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发现是不是很简单,在这里感谢斯巴达的无私赐教.

提供动态头部信息
如果按照上述方法进行抓取,很多时候会因为提交信息过于单一,被服务器认为是机器爬虫进行拒绝.
那我们是不是有一些更为智能的方法提交一些动态的数据,答案肯定也是肯定的.而且很简单,直接上代码!

'''
@本程序是用来动态提交Header信息
@random 动态库,详情请参考<https://docs.python.org/2/library/random.html>
'''
 
# coding=utf-8
import urllib2
import random
 
url = 'http://www.lifevc.com/'
 
my_headers = [
  'Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.04506.648)',
  'Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727; InfoPath.1',
  'Mozilla/4.0 (compatible; GoogleToolbar 5.0.2124.2070; Windows 6.0; MSIE 8.0.6001.18241)',
  'Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)',
  'Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0; Sleipnir/2.9.8)',
  #因篇幅关系,此处省略N条
]
 
random_header = random.choice(headers)
# 可以通过print random_header查看提交的header信息
req = urllib2.Request(url)
req.add_header("User-Agent", random_header)
req.add_header('Host', 'blog.csdn.net')
req.add_header('Referer', 'http://blog.csdn.net')
req.add_header('GET', url)
content = urllib2.urlopen(req).read()
print content

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其实很简单,这样我们就完成了对代码的一些优化.

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