十三、CSS选择器：BeautifulSoup4
作者：互联网
（1）和lxml一样,Beautifu 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。
（2）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。
（3）BeautifulSoup用来解析HTML比较简单，API非常人性化，支持CSS选择器、python标准库中的HTML解析器，也支持lxml的XML解析器。
安装：`pip install beautifusoup4`
官方文档：https://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 |
1、示例
from bs4 import BeautifulSoup
html = """
<html><head><title>The Dormouse's story</title></head>
<body>
<p class="title" name="dromouse"><b>The Dormouse's story</b></p>
<p class="story">Once upon a time there were three little sisters; and their names were
<a href="http://example.com/elsie" class="sister" id="link1"><!-- Elsie --></a>,
<a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
<a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
and they lived at the bottom of a well.</p>
<p class="story">...</p>
"""
# 创建Beautiful Soup对象
soup = BeautifulSoup(html,'lxml')
# 打开本地HTML文件的方式来创建对象
# soup = BeautifulSoup(open('index.html'))
# 格式化输出soup对象的内容
print(soup.prettify())
运行结果：
<html>
<head>
<title>
The Dormouse's story
</title>
</head>
<body>
<p class="title" name="dromouse">
<b>
The Dormouse's story
</b>
</p>
<p class="story">
Once upon a time there were three little sisters; and their names were
<a class="sister" href="http://example.com/elsie" id="link1">
<!-- Elsie -->
</a>
,
<a class="sister" href="http://example.com/lacie" id="link2">
Lacie
</a>
and
<a class="sister" href="http://example.com/tillie" id="link3">
Tillie
</a>
;
and they lived at the bottom of a well.
</p>
<p class="story">
...
</p>
</body>
</html>
如果没有显示地指定解析器，会默认使用这个系统的最佳可用HTML解析器（'lxml'）。当在另一个系统中运行这段代码，或者在不同的虚拟环境中，使用不同的解析器会造成不同行为。
可以通过`soup=BeautifuSoup(html,'lxml')`方式指定lxml解析器。
2、四大对象种类
Beautifu 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：（1）Tag（2）NavigableString（3）BeautifulSoup（4）Comment
2.1 Tag
<head><title>The Dormouse's story</title></head> <a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a> <p class="title" name="dromouse"><b>The Dormouse's story</b></p>
Tag是，HTML中的一个个标签（即上面代码中的`title`、`head`、`a`、`p`等等HTML标签）加上里面包括的内容。
from bs4 import BeautifulSoup
html = """
<html><head><title>The Dormouse's story</title></head>
<body>
<p class="title" name="dromouse"><b>The Dormouse's story</b></p>
<p class="story">Once upon a time there were three little sisters; and their names were
<a href="http://example.com/elsie" class="sister" id="link1"><!-- Elsie --></a>,
<a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
<a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
and they lived at the bottom of a well.</p>
<p class="story">...</p>
"""
# 创建Beautiful Soup对象
soup = BeautifulSoup(html,'lxml')
# 打开本地HTML文件的方式来创建对象
# soup = BeautifulSoup(open('index.html'))
# 格式化输出soup对象的内容
# print(soup.prettify())
print(soup.title)
# <title>The Dormouse's story</title>
print(soup.head)
# <head><title>The Dormouse's story</title></head>
print(soup.a)
# <a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>
print(type(soup.a))
# <class 'bs4.element.Tag'>
# 通过soup加标签名获取这些标签的内容，这些对象的类型是bs4.element.Tag
# 通过这种方法查找的是在所有内容中的第一个符合要求的标签。
# 对于Tag，它本身有两个重要的属性，即name和attrs
print(soup.name)
# [document]
# soup对象本身比较特殊，它的name即为[document]
print(soup.head.name)
# head
# 对于其他内部标签，输出的值便为标签本身的名称
print(soup.p.attrs)
# {'class': ['title'], 'name': 'dromouse'}
# 在这里，我们把p标签的所有属性打印了出来，得到的类型是一个字典
print(soup.p['class'])
# [‘title’] 获取属性的值
# 等同下列get方法
print(soup.p.get('class'))
# ['title'] 获取属性的值
soup.p['class'] = 'newClass'
# 对p标签下的class属性的内容进行修改
print(soup.p)
# <p class="newClass" name="dromouse"><b>The Dormouse's story</b></p>
del soup.p['class'] # 还可以对这个属性进行删除
print soup.p
# <p name="dromouse"><b>The Dormouse's story</b></p>2.2 NavigableString
通过.string的方式即可获取标签内部的文字
print soup.p.string # The Dormouse's story print type(soup.p.string) # In [13]: <class 'bs4.element.NavigableString'>
2.3 BeautifulSoup
BeautifulSoup对象表示的是一个文档的内容，可以把它当做是一个特殊的Tag对象，可以分别获取它的类型，名称以及属性。
print(type(soup.name))
# <class 'str'>
print(soup.name)
# [document]
print(soup.attrs)
# {} 文档本身的属性为空
2.4 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。
print(soup.a) # <a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a> print(soup.a.string) # Elsie print(type(soup.a.string)) # <class 'bs4.element.Comment'>
注意Comment和NavigableString对象的区别，当HTML标签的.string中有注释时，忽视注释符号，返回其中的内容，这时它是一个Comment对象；当没有注释时，返回其中的内容，这时它是一个NavigableString对象。
3、遍历文档树
3.1 直接子节点：`.contents`，`.children`属性
（1）`.content`属性
Tag的`.contents`属性可以将Tag的子节点以列表的方式输出
print(soup.body.contents) # tag的.contents属性可以将tag的子节点以列表的方式输出 """ ['\n', <p class="title" name="dromouse"><b>The Dormouse's story</b></p>, '\n', <p class="story">Once upon a time there were three little sisters; and their names were <a class = "sister" href = "http://example.com/elsie" id = "link1" > <!-- Elsie - -> < /a > , <a class = "sister" href = "http://example.com/lacie" id = "link2" > Lacie < /a > and <a class = "sister" href = "http://example.com/tillie" id = "link3" > Tillie < /a >; and they lived at the bottom of a well. < /p > , '\n', < p class = "story" > ... < /p > , '\n'] """ # 输出方式为列表，可以用列表索引来获取它的某一个元素 print(soup.body.contents[1]) #<p class="title" name="dromouse"><b>The Dormouse's story</b></p>
（2）`.children`属性
Tag的`.children`属性，返回一个list生成器对象。
print(soup.body.children)
# <list_iterator object at 0x7f55adea9d68>
for child in soup.body.children:
print(child)
# 输出结果
"""
<p class="title" name="dromouse"><b>The Dormouse's story</b></p>
<p class="story">Once upon a time there were three little sisters; and their names were
<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>,
<a class="sister" href="http://example.com/lacie" id="link2">Lacie</a> and
<a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>;
and they lived at the bottom of a well.</p>
<p class="story">...</p>
"""
3.2 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，返回一个生成器对象。
print(soup.descendants)
# <generator object descendants at 0x7f98e70050f8>
for child in soup.descendants:
print(child)
"""
<html><head><title>The Dormouse's story</title></head>
<body>
<p class="title" name="dromouse"><b>The Dormouse's story</b></p>
<p class="story">Once upon a time there were three little sisters; and their names were
<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>,
<a class="sister" href="http://example.com/lacie" id="link2">Lacie</a> and
<a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>;
and they lived at the bottom of a well.</p>
<p class="story">...</p>
</body></html>
<head><title>The Dormouse's story</title></head>
<title>The Dormouse's story</title>
The Dormouse's story
<body>
<p class="title" name="dromouse"><b>The Dormouse's story</b></p>
<p class="story">Once upon a time there were three little sisters; and their names were
<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>,
<a class="sister" href="http://example.com/lacie" id="link2">Lacie</a> and
<a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>;
and they lived at the bottom of a well.</p>
<p class="story">...</p>
</body>
<p class="title" name="dromouse"><b>The Dormouse's story</b></p>
<b>The Dormouse's story</b>
The Dormouse's story
<p class="story">Once upon a time there were three little sisters; and their names were
<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>,
<a class="sister" href="http://example.com/lacie" id="link2">Lacie</a> and
<a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>;
and they lived at the bottom of a well.</p>
Once upon a time there were three little sisters; and their names were
<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>
Elsie
,
<a class="sister" href="http://example.com/lacie" id="link2">Lacie</a>
Lacie
and
<a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>
Tillie
;
and they lived at the bottom of a well.
<p class="story">...</p>
...
"""
3.3 节点内容：`.string`属性
如果tag只有一个 NavigableString 类型子节点,那么这个tag可以使用 .string 得到子节点。如果一个tag仅有一个子节点,那么这个tag也可以使用 .string 方法,输出结果与当前唯一子节点的 .string 结果相同。
print soup.head.string #The Dormouse's story print soup.title.string #The Dormouse's story
4、搜索文档树
4.1 find_all(name,attrs,recursive,text,**kwargs)
4.1.1 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。
（1）传字符串
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
print(type(soup.find_all('p')))
# <class 'bs4.element.ResultSet' >
print(soup.find_all('p'))
"""
<p class="title" name="dromouse"><b>The Dormouse's story</b></p>, <p class="story">Once upon a time there were three little sisters; and their names were
<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>,
<a class="sister" href="http://example.com/lacie" id="link2">Lacie</a> and
<a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>;
and they lived at the bottom of a well.</p>, <p class="story">...</p>]
"""
（2）传入正则表达式
如果传入正则表达式作为参数，Beautiful Soup会通过正则表达式的macth()来匹配内容。
# 找出所有以b开头的标签
import re
for tag in soup.find_all(re.compile('^b')):
print(tag.name)
# body
# b
（3）传列表
如果传入列表参数，Beautiful Soup会将把与列表中任一元素匹配的内容返回。
print(soup.find_all(["a",'p'])) """ [<p class="title" name="dromouse"><b>The Dormouse's story</b></p>, <p class="story">Once upon a time there were three little sisters; and their names were <a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>, <a class="sister" href="http://example.com/lacie" id="link2">Lacie</a> and <a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>; and they lived at the bottom of a well.</p>, <a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>, <a class="sister" href="http://example.com/lacie" id="link2">Lacie</a>, <a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>, <p class="story">...</p>] """
4.1.2 keyword参数
soup.find_all(id='link2') # [<a class="sister" href="http://example.com/lacie" id="link2">Lacie</a>]
4.1.3 text参数
通过text参数可以搜索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、正则表达式及列表
import re
print(soup.find_all(text='Tillie'))
print(soup.find_all(text=["Tillie","Elsie","Lacie"]))
print(soup.find_all(text=re.compile("Dormouse")))
"""
['Tillie']
['Lacie', 'Tillie']
["The Dormouse's story", "The Dormouse's story"]
"""
4.2 CSS选择器
写CSS时，标签名不加任何修饰，类名前加`.`，id名前加`#`
用soup.select()，返回类型是list
4.2.1 通过标签名查找
print(soup.select("title"))
# [<title>The Dormouse's story</title>]
print(soup.select("a"))
"""
[<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>, <a class="sister" href="http://example.com/lacie" id="link2">Lacie</a>, <a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>]
"""
print(soup.select('b'))
# [<b>The Dormouse's story</b>]
4.2.2 通过类名查找
print(soup.select(".sister"))
"""
[<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>, <a class="sister" href="http://example.com/lacie" id="link2">Lacie</a>, <a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>]
"""
4.2.3 通过id名查找
print(soup.select("#link1"))
# [<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>]
4.2.4 组合查找
组合查找即和写css文件时，标签名与类名、id名进行组合的原理是一样的，其各之间需要用空格分开。
print(soup.select("p #link1"))
# [<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>]
直接子标签查找，则使用`>`分隔
print(soup.select("head > title"))
#[<title>The Dormouse's story</title>]
4.2.5 属性查找
查找时还可以加入属性元素，属性需要用中括号括起来，注意属性与标签属于同一节点，所以中间不能加空格，否则会无法匹配到。
print(soup.select('a[class="sister"]'))
#[<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>, <a class="sister" href="http://example.com/lacie" id="link2">Lacie</a>, <a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>]
print(soup.select('a[href="http://example.com/elsie"]'))
#[<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>]
同样，属性仍然可以与上述查找方式组合，不在同一节点的空格隔开，同一节点的不加空格
print(soup.select('p a[href="http://example.com/elsie"]'))
#[<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>]
4.2.6 获取内容
select 方法返回的结果都是列表形式，可以遍历形式输出，然后用 get_text() 方法来获取它的内容。
soup = BeautifulSoup(html,'lxml')
print(type(soup.select("title")))
# <class 'bs4.element.ResultSet'>
print(soup.select('title')[0])
# <title>The Dormouse's story</title>
print(soup.select("title")[0].get_text())
# The Dormouse's story
for title in soup.select("title"):
print(title.get_text())
# The Dormouse's story
标签：Tillie,story,Dormouse,soup,were,print,BeautifulSoup4,选择器,CSS 来源： https://www.cnblogs.com/nuochengze/p/12863045.html