Python爬虫之BeautifulSoup4
爬虫——BeautifulSoup4解析器
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。
其相较与正则而言，使用更加简单。
示例：
首先必须要导入bs4库
#!/usr/bin/python3# -*- coding:utf-8 -*-
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，指定lxml解析器
soup = BeautifulSoup(html, "lxml")
# 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>
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四大对象种类
BeautifulSoup将复杂的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
1.Tag
Tag 通俗点讲就是HTML中的一个个标签，例如：
<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>
上面title head a p 等等HTML标签加上里面包括的内容就是Tag，那么试着使用BeautifulSoup来获取Tags：
#!/usr/bin/python3# -*- coding:utf-8 -*-
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，指定lxml解析器
soup = BeautifulSoup(html, "lxml")
# # 打印title标签
print(soup.title)
# 打印head标签
print(soup.head)
# 打印a标签
print(soup.a)
# 打印p标签
print(soup.p)
# 打印soup.p的类型
print(type(soup.p))
运行结果
<title>The Dormouse's story</title><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>
<class 'bs4.element.Tag'>
我们可以利用soup加标签名轻松地获取这些标签内容，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。如果需要查询所有的标签，后面会进行介绍。
对于Tag，它有两个重要的属性，就是name和attrs。
#!/usr/bin/python3# -*- coding:utf-8 -*-
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，指定lxml解析器
soup = BeautifulSoup(html, "lxml")
# soup对象比较特殊，它的name为[document]
print(soup.name)
# 对于其他内部标签，输出的值便为标签本身的名称
print(soup.head.name)
# 打印p标签的所有属性，其类型是一个字典
print(soup.p.attrs)
# 打印p标签的class属性
print(soup.p['class'])
# 还可以利用get方法获取属性，传入属性的名称，与上面的方法等价
print(soup.p.get('class'))
print(soup.p)
# 修改属性
soup.p['class'] = "newClass"
print(soup.p)
# 删除属性
del soup.p['class']
print(soup.p)
运行结果
[document]head
{'class': ['title'], 'name': 'dromouse'}
['title']
['title']
<p class="title" name="dromouse"><b>The Dormouse's story</b></p>
<p class="newClass" name="dromouse"><b>The Dormouse's story</b></p>
<p name="dromouse"><b>The Dormouse's story</b></p>
2.NavigableString
既然我们已经得到了标签的内容，那么问题来了，我们想要获取标签内部的文字怎么办呢？很简单，用.string即可，例如：
#!/usr/bin/python3# -*- coding:utf-8 -*-
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，指定lxml解析器
soup = BeautifulSoup(html, "lxml")
# 打印p标签的内容
print(soup.p.string)
# 打印soup.p.string的类型
print(type(soup.p.string))
运行结果
The Dormouse's story<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。
#!/usr/bin/python3# -*- coding:utf-8 -*-
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，指定lxml解析器
soup = BeautifulSoup(html, "lxml")
# 类型
print(type(soup.name))
# 名称
print(soup.name)
# 属性
print(soup.attrs)
运行结果
<class 'str'>[document]
{}
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。
#!/usr/bin/python3# -*- coding:utf-8 -*-
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，指定lxml解析器
soup = BeautifulSoup(html, "lxml")
print(soup.a)
print(soup.a.string)
print(type(soup.a.string))
运行结果
<a class="sister" href="http://example.com/elsie" id="link1"><!-- Elsie --></a>Elsie
<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。
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