Python 爬虫从入门到进阶之路（十二） - 好文

之前的文章我们介绍了 re 模块和 lxml 模块来做爬虫，本章我们再来看一个 bs4 模块来做爬虫。

和 lxml 一样，Beautiful Soup 也是一个HTML/XML的解析器，主要的功能也是如何解析和提取 HTML/XML 数据。

lxml 只会局部遍历，而Beautiful Soup 是基于HTML
DOM的，会载入整个文档，解析整个DOM树，因此时间和内存开销都会大很多，所以性能要低于lxml。

BeautifulSoup 用来解析 HTML 比较简单，API非常人性化，支持CSS选择器
<http://www.w3school.com.cn/cssref/css_selectors.asp>、Python标准库中的HTML解析器，也支持
lxml 的 XML解析器。

Beautiful Soup 3 目前已经停止开发，推荐现在的项目使用Beautiful Soup 4。使用 pip 安装即可：pip install
beautifulsoup4

官方文档：http://beautifulsoup.readthedocs.io/zh_CN/v4.4.0
<http://beautifulsoup.readthedocs.io/zh_CN/v4.4.0/>

抓取工具速度使用难度安装难度
正则最快困难无（内置）
BeautifulSoup 慢最简单简单
lxml 快简单一般
首先必须要导入 bs4 库
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 18 # 打开本地 HTML 文件的方式来创建对象 19 # soup =
BeautifulSoup(open('index.html'), "lxml") 20 21 # 格式化输出 soup 对象的内容 22 print
(soup.prettify())
运行结果：
1 <html> 2 <body> 3 <div> 4 <ul> 5 <li class="item-0"> 6 <a href
="link1.html"> 7 first item 8 </a> 9 </li> 10 <li class="item-1"> 11 <a href
="link2.html"> 12 second item 13 </a> 14 </li> 15 <li class="item-inactive"> 16
<a href="link3.html"> 17 <span class="bold"> 18 third item 19 </span> 20 </a>
21 </li> 22 <li class="item-1"> 23 <a href="link4.html"> 24 fourth item 25 </a>
26 </li> 27 <li class="item-0"> 28 <a href="link5.html"> 29 fifth item 30 </a>
31 </li> 32 </ul> 33 </div> 34 </body> 35 </html>
四大对象种类

Beautiful Soup将复杂HTML文档转换成一个复杂的树形结构,每个节点都是Python对象,所有对象可以归纳为4种:

* Tag
* NavigableString
* BeautifulSoup
* Comment
1. Tag

Tag 通俗点讲就是 HTML 中的一个个标签，例如：
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 18 print(soup.li) # <li class="item-0"><a
href="link1.html">first item</a></li> 19 print(soup.a) # <a
href="link1.html">first item</a> 20 print(soup.span) # <span class="bold">third
item</span> 21 print(soup.p) # None 22 print(type(soup.li)) # <class
'bs4.element.Tag'>
我们可以利用 soup 加标签名轻松地获取这些标签的内容，这些对象的类型是bs4.element.Tag
。但是注意，它查找的是在所有内容中的第一个符合要求的标签。如果要查询所有的标签，后面会进行介绍。

对于 Tag，它有两个重要的属性，是 name 和 attrs
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 18 print(soup.li.attrs) # {'class': ['item-0']} 19
print(soup.li["class"]) # ['item-0'] 20 print(soup.li.get('class')) # ['item-0']
21 22 print(soup.li) # <li class="item-0"><a href="link1.html">first
item</a></li> 23 soup.li["class"] = "newClass" # 可以对这些属性和内容等等进行修改 24 print
(soup.li)# <li class="newClass"><a href="link1.html">first item</a></li> 25 26
del soup.li['class'] # 还可以对这个属性进行删除 27 print(soup.li) # <li><a
href="link1.html">first item</a></li>

2. NavigableString

既然我们已经得到了标签的内容，那么问题来了，我们要想获取标签内部的文字怎么办呢？很简单，用 .string 即可，例如
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 18 print(soup.li.string) # first item 19 print
(soup.a.string)# first item 20 print(soup.span.string) # third item 21 #
print(soup.p.string) # AttributeError: 'NoneType' object has no attribute
'string' 22 print(type(soup.li.string)) # <class 'bs4.element.NavigableString'>
3. BeautifulSoup

BeautifulSoup 对象表示的是一个文档的内容。大部分时候,可以把它当作 Tag 对象，是一个特殊的
Tag，我们可以分别获取它的类型，名称，以及属性来感受一下
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 18 print(soup.name) # [document] 19 print
(soup.attrs)# {}, 文档本身的属性为空 20 print(type(soup.name)) # <class 'str'>
4. Comment

Comment 对象是一个特殊类型的 NavigableString 对象，其输出的内容不包括注释符号。
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <a
class="sister" href="http://example.com/elsie" id="link1"></a> 6
</div> 7 """ 8 9 # 创建 Beautiful Soup 对象 10 soup = BeautifulSoup(html, "lxml")
11 12 print(soup.a) # <a class="sister" href="http://example.com/elsie"
id="link1"></a> 13 print(soup.a.string) # Elsie 14 print
(type(soup.a.string))# <class 'bs4.element.Comment'>
a 标签里的内容实际上是注释，但是如果我们利用 .string 来输出它的内容时，注释符号已经去掉了。

遍历文档树

1. 直接子节点：.contents .children 属性

.content

tag 的 .content 属性可以将tag的子节点以列表的方式输出，输出方式为列表，我们可以用列表索引来获取它的某一个元素
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 18 print(soup.li.contents) # [<a
href="link1.html">first item</a>] 19 print(soup.li.contents[0]) # <a
href="link1.html">first item</a>
.children

它返回的不是一个 list，不过我们可以通过遍历获取所有子节点。

我们打印输出 .children 看一下，可以发现它是一个 list 生成器对象
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 18 print(soup.ul.children) # <list_iterator
object at 0x106388a20> 19 for child in soup.ul.children: 20 print(child)
输出结果：
1 <li class="item-0"><a href="link1.html">first item</a></li> 2 3 4 <li
class="item-1"><a href="link2.html">second item</a></li> 5 6 7 <li class
="item-inactive"><a href="link3.html"><span class="bold">third item</span></a></
li> 8 9 10 <li class="item-1"><a href="link4.html">fourth item</a></li> 11 12
13 <li class="item-0"><a href="link5.html">fifth item</a></li>
2. 所有子孙节点: .descendants 属性

.contents 和 .children 属性仅包含tag的直接子节点，.descendants 属性可以对所有tag的子孙节点进行递归循环，和
children类似，我们也需要遍历获取其中的内容。
1 for child in soup.ul.descendants: 2 print(child)
运行结果：
1 <li class="item-0"><a href="link1.html">first item</a></li> 2 <a href
="link1.html">first item</a> 3 first item 4 5 6 <li class="item-1"><a href
="link2.html">second item</a></li> 7 <a href="link2.html">second item</a> 8
second item 9 10 11 <li class="item-inactive"><a href="link3.html"><span class
="bold">third item</span></a></li> 12 <a href="link3.html"><span class="bold">
third item</span></a> 13 <span class="bold">third item</span> 14 third item 15
16 17 <li class="item-1"><a href="link4.html">fourth item</a></li> 18 <a href
="link4.html">fourth item</a> 19 fourth item 20 21 22 <li class="item-0"><a href
="link5.html">fifth item</a></li> 23 <a href="link5.html">fifth item</a> 24
fifth item

搜索文档树

1.find_all(name, attrs, recursive, text, **kwargs)

1）name 参数

name 参数可以查找所有名字为 name 的 tag，字符串对象会被自动忽略掉

A.传字符串

最简单的过滤器是字符串.在搜索方法中传入一个字符串参数,Beautiful Soup会查找与字符串完整匹配的内容,下面的例子用于查找文档中所有的<span>
标签:
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 print(soup.find_all('span')) # [<span
class="bold">third item</span>]
B.传正则表达式

如果传入正则表达式作为参数,Beautiful Soup会通过正则表达式的 match() 来匹配内容.下面例子中找出所有以 s 开头的标签,这表示
<span> 标签都应该被找到
1 from bs4 import BeautifulSoup 2 import re 3 4 html = """ 5 <div> 6
<ul> 7 <li class="item-0"><a href="link1.html">first item</a></li> 8 <li
class="item-1"><a href="link2.html">second item</a></li> 9 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li>10 <li class="item-1"><a href="link4.html">fourth
item</a></li>11 <li class="item-0"><a href="link5.html">fifth item</a></li> 12
</ul>13 </div> 14 """ 15 16 # 创建 Beautiful Soup 对象 17 soup =
BeautifulSoup(html,"lxml") 18 for tag in soup.find_all(re.compile("^s")): 19
print(tag) 20 # <span class="bold">third item</span>
C.传列表

如果传入列表参数,Beautiful Soup会将与列表中任一元素匹配的内容返回.下面代码找到文档中所有 <a> 标签和 <span> 标签:
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 print(soup.find_all(["a", "span"])) 18 # [<a
href="link1.html">first item</a>, <a href="link2.html">second item</a>, <a
href="link3.html"><span class="bold">third item</span></a>, <span
class="bold">third item</span>, <a href="link4.html">fourth item</a>, <a
href="link5.html">fifth item</a>]
2）keyword 参数
1 from bs4 import BeautifulSoup 2 3 html = """ 4 <div> 5 <ul> 6 <li
class="item-0"><a href="link1.html">first item</a></li> 7 <li
class="item-1"><a href="link2.html">second item</a></li> 8 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li> 9 <li class="item-1"><a href="link4.html">fourth
item</a></li>10 <li class="item-0"><a href="link5.html">fifth item</a></li> 11
</ul>12 </div> 13 """ 14 15 # 创建 Beautiful Soup 对象 16 soup =
BeautifulSoup(html,"lxml") 17 print(soup.find_all(href='link1.html')) # [<a
href="link1.html">first item</a>]
3）text 参数

通过 text 参数可以搜搜文档中的字符串内容，与 name 参数的可选值一样, text 参数接受字符串 , 正则表达式 , 列表
1 from bs4 import BeautifulSoup 2 import re 3 4 html = """ 5 <div> 6
<ul> 7 <li class="item-0"><a href="link1.html">first item</a></li> 8 <li
class="item-1"><a href="link2.html">second item</a></li> 9 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li>10 <li class="item-1"><a href="link4.html">fourth
item</a></li>11 <li class="item-0"><a href="link5.html">fifth item</a></li> 12
</ul>13 </div> 14 """ 15 16 # 创建 Beautiful Soup 对象 17 soup =
BeautifulSoup(html,"lxml") 18 print(soup.find_all(text="first item")) # ['first
item'] 19 print(soup.find_all(text=["first item", "second item"])) # ['first
item', 'second item'] 20 print(soup.find_all(text=re.compile("item"))) #
['first item', 'second item', 'third item', 'fourth item', 'fifth item']
CSS选择器

这就是另一种与 find_all 方法有异曲同工之妙的查找方法.

*
写 CSS 时，标签名不加任何修饰，类名前加.，id名前加#

*
在这里我们也可以利用类似的方法来筛选元素，用到的方法是 soup.select()，返回类型是 list

（1）通过标签名查找
1 from bs4 import BeautifulSoup 2 import re 3 4 html = """ 5 <div> 6
<ul> 7 <li class="item-0"><a href="link1.html">first item</a></li> 8 <li
class="item-1"><a href="link2.html">second item</a></li> 9 <li
class="item-inactive"><a href="link3.html"><span class="bold">third
item</span></a></li>10 <li class="item-1"><a href="link4.html">fourth
item</a></li>11 <li class="item-0"><a href="link5.html">fifth item</a></li> 12
</ul>13 </div> 14 """ 15 16 # 创建 Beautiful Soup 对象 17 soup =
BeautifulSoup(html,"lxml") 18 print(soup.select('span')) # [<span
class="bold">third item</span>]
（2）通过类名查找
1 print(soup.select('.item-0')) 2 # [<li class="item-0"><a
href="link1.html">first item</a></li>, <li class="item-0"><a
href="link5.html">fifth item</a></li>]
（3）通过 id 名查找
print(soup.select('#item-0')) # []
（4）组合查找
1 print(soup.select('li.item-0')) 2 # [<li class="item-0"><a
href="link1.html">first item</a></li>, <li class="item-0"><a
href="link5.html">fifth item</a></li>] 3 print(soup.select('li.item-0>a')) 4 #
[<a href="link1.html">first item</a>, <a href="link5.html">fifth item</a>]
（5）属性查找
1 print(soup.select('a[href="link1.html"]')) # [<a href="link1.html">first
item</a>]
(6) 获取内容
1 for text in soup.select('li'): 2 print(text.get_text()) 3 """ 4 first item 5
second item6 third item 7 fourth item 8 fifth item 9 """

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