1、BeautifulSoup4库简介
What is beautifulsoup ?
答：一个可以用来从HTML 和 XML中提取数据的网页解析库，支持多种解析器（代替正则的复杂用法）
2、安装
pip3 install beautifulsoup4
3、用法详解
（1）、解析器性能分析（第一个参数markup-要解析的目标代码，第二个参数为解析器）
（2）、使用方法（独孤九剑）
1、总诀式：
#author: "xian"
#date: 2018/5/7
#以下为爱丽丝梦游仙境的部分代码
html = """
<html><head><title>The Dormouse's story</title></head>
<body>
<p class="title"><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>
"""
#小试牛刀
from bs4 import BeautifulSoup #从bs4库导入BeautifulSoup类 soup = BeautifulSoup(html,'lxml') #构造名为soup的对象
print(soup.prettify()) #prettify修饰()方法：格式化代码也就是让各位小伙伴释放眼睛压力哈哈！
print(soup.a) #选中a标签
print(soup.a['class'])#打印a标签名为class的属性值
print(soup.a.name) #打印a 标签的名字 soup.a.parent.name 找到a标签的老子
print(soup.a.string) #小伙伴们猜猜看这是干什么？ 答：打印a标签的文本
print(soup.find_all('a')) #找到所有的a标签
print(soup.find(id="link3"))#找到id属性值为link3的标签 #找链接
for link in soup.find_all('a'):
print(link.get('href')) #遍历所有名为a的标签并得到其链接
#找文本
print(soup.a.get_text()) #获取a标签的文本当然小伙伴们可以任意指定想要的内容 #上面的输出
'''<html>
<head>
<title>
The Dormouse's story
</title>
</head>
<body>
<p class="title">
<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>
<a class="sister" href="http://example.com/elsie" id="link1">Elsie</a>
['sister']
a
Elsie
[<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>]
<a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>
http://example.com/elsie
http://example.com/lacie
http://example.com/tillie'''
其他的小伙伴们可以根据需要获取想要的内容，掌握方法即可，具体可参见官网：https://www.crummy.com/software/BeautifulSoup/bs4/doc.zh/
2、破剑式
#author: "xian"
#date: 2018/5/7
html = """
<html>
<head>
<title>The Dormouse's story</title>
</head>
<body>
<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">
<span>Elsie</span>
</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>
"""
#子节点及子孙节点（老子节点与祖宗节点的选择）的选择
from bs4 import BeautifulSoup soup = BeautifulSoup(html,'lxml')
print(soup.p.contents) #contents方法将得到的结果以列表形式输出
print(soup.p.children) #是一个迭代器对象，需要用for循环才能得到器内容 children 只后期子节点
for i,child in enumerate(soup.p.children): #enumerate() 函数用于将一个可遍历的数据对象(如列表、元组或字符串)组合为一个索引序列，同时列出数据和数据下标，一般用在 for 循环当中。
print(i,child) #接受index 和内特
print(soup.p.descendants) #descendants 获取所有的儿子和孙子后代节点
for i,child in enumerate(soup.p.descendants):
print(i,child) #上面的输出结果
'''['\n Once upon a time there were three little sisters; and their names were\n ', < a
class ="sister" href="http://example.com/elsie" id="link1" >
< span > Elsie < / span >
< / a >, '\n', < a class ="sister" href="http://example.com/lacie" id="link2" > Lacie < / a >, '\n and\n ', < a class ="sister" href="http://example.com/tillie" id="link3" > Tillie < / a >, '\n and they lived at the bottom of a well\n ']
< list_iterator object at 0x00000156B2E76EF0 >
0
Once upon a time there were three little sisters; and their names were 1 < a class ="sister" href="http://example.com/elsie" id="link1" >
< span > Elsie < / span >
< / a >
2 3 < a class ="sister" href="http://example.com/lacie" id="link2" > Lacie < / a >
4
and 5 < a class ="sister" href="http://example.com/tillie" id="link3" > Tillie < / a >
6
and they lived at the bottom of a well < generator object descendants at 0x00000156B08910F8 >
0 Once upon a time there were three little sisters; and their names were 1 < a class ="sister" href="http://example.com/elsie" id="link1" >
< span > Elsie < / span >
< /a >
2 3 < span > Elsie < / span >
4 Elsie
5 6 7 < a class ="sister" href="http://example.com/lacie" id="link2" > Lacie < / a >
8 Lacie
9
and 10 < a class ="sister" href="http://example.com/tillie" id="link3" > Tillie < / a >
11 Tillie
12
and they lived at the bottom of a well''' #老子节点和祖宗节点方法介绍 children -- parent / descendants -- parents 小伙伴们模仿上面的可是动手试试
#兄弟节点的获取 方法为：next_siblings：获取当前对象后面的兄弟节点 previous_siblings:获取当前对象前面的兄弟节点，小伙伴们可以试试
3、破刀式
#author: "xian"
#date: 2018/5/7
#搜索文档内容 find_all() 和find()
html = """
<html><head><title>The Dormouse's story</title></head> <p class="title"><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>
"""
from bs4 import BeautifulSoup
import re soup = BeautifulSoup(html,'lxml')
#(1)、find_all( name , attrs , recursive , text , **kwargs )
#name参数用法详解（text参数的使用同name类似如soup.find_all(text=["Tillie", "Elsie", "Lacie"])只返回内容，小伙伴们可查阅官方文档：https://www.crummy.com/software/BeautifulSoup/bs4/doc.zh/）
print(soup.find_all('head')) #查找head标签
print(soup.find_all(id='link2')) #查找id='link2'的标签
print(soup.find_all(href=re.compile("(\w+)"))) #查找所有包含href属性包含字母数字的标签
print(soup.find_all(href=re.compile("(\w+)"), id='link1')) #多重过滤
#搜索指定名字的属性时可以使用的参数值包括 字符串 , 正则表达式 , 列表, True #attrs参数用法详解
print(soup.find_all(attrs={'id':'link2'})) #attrs参数以key-value形式传入值 /返回列表类型 #（2）find( name , attrs , recursive , text , **kwargs )用法同find_all 类似只不过它只返回一个值，小伙伴们可以查找官方用法 #（3）其他方法汇总：(小伙伴们了解即可具体碰到查文档)
#find_parents() 和find_parent() 返回祖宗节点 和 返回老子节点
#find_next_siblings() 和 find_next_sibling() 返回后面所有的兄弟节点 和 返回后面第一个兄弟节点
#find_previous_siblings() 和 find_previous_sibling() 返回前面所有的兄弟节点 和 返回前面第一个兄弟节点
#find_all_next() 和 find_next() 返回节点后满足条件所有的节点 和 返回第一个满足条件的节点
#find_all_previous() 和 find_previous() 返回节点前满足条件所有的节点 和 返回第一个满足条件的节点 #上面的输出结果：
'''
[<head><title>The Dormouse's story</title></head>]
[<a class="sister" href="http://example.com/lacie" id="link2">Lacie</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>]
[<a class="sister" href="http://example.com/elsie" id="link1">Elsie</a>]
[<a class="sister" href="http://example.com/lacie" id="link2">Lacie</a>]
<class 'bs4.element.ResultSet'>
'''
4、破枪式
#author: "xian"
#date: 2018/5/7
#CSS选择器详解（通过select()传入css选择器即可成功选择）
html = """
<html><head><title>The Dormouse's story</title></head>
<body>
<p class="title"><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>
"""
from bs4 import BeautifulSoup soup = BeautifulSoup(html,'lxml')
print(soup.select('.title')) #选择class属性为title的标签 css选择器使用请小伙伴们查看官网
#再来一例
print(soup.select('p a#link1'))# 选择p标签下的a下的id属性为link1的标签
print(soup.select('a')[1]) #做一个切片拿到第二个a标签
#获取内容
print(soup.select('a')[1].get_text()) #上面的输出：
'''
[<p class="title"><b>The Dormouse's story</b></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>
laci
33 '''
通过以上的实验，小伙伴们对bs4库是否有了一定的了解，赶紧行动起来，试试学习的效果吧！
总结：
1.建议小伙伴使用lxml解析器
2.多用find_all()和find()
3.css的select()方法掌握下
4.多练习，勤能补拙，孰能生巧，才能渐入化境！