中文文档：https://beautifulsoup.readthedocs.io/zh_CN/v4.4.0/#id49
基本用法
实例1：
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>
"""
soup = BeautifulSoup(html,'lxml')
print(soup.prettify())
print(type(soup))
print(soup.title.string)结果：
说明1：
可以看到，我们输入的并不是一个完整的HTML字符串，缺少了</body>等标签，我们初始化BeautifulSoup时完成了自动更正格式。
说明2：
soup.prettify()方法将xml/html标签独占一行：https://beautifulsoup.readthedocs.io/zh_CN/v4.4.0/#id49
节点选择器
实例2：提取信息和嵌套选择
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>
"""
soup = BeautifulSoup(html,'lxml')
#获取节点#
print('soutp.title:',soup.title) #选择元素
print('soup.a:',soup.a) #取得的只是第一个a节点
#获取名称#
print('soup.title.name:',soup.title.name) # .name获取名称
#获取属性#
print('soup.p.attrs:',soup.p.attrs) # .attrs获取所有属性
print('soup.p.attrs["name"]:',soup.p.attrs['name']) #获取限定属性
print('soup.p["name]:',soup.p['name']) #更简单的写法
#获取内容#
print('soup.p.string:',soup.p.string)
#嵌套选择#
print('soup.head.title:',soup.head.title)结果
实例3：子节点
from bs4 import BeautifulSoup
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"><!--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>
"""
soup = BeautifulSoup(html,'lxml')
for i,content in enumerate(soup.p.contents):
print(i,content)
for i,child in enumerate(soup.p.children):
print(i,child)
#遍历输出是一致的。
print('contents:',soup.p.contents)
print('children:',soup.p.children)
print('type of contents:',type(soup.p.contents))
print('type of children:',type(soup.p.children))结果：
说明：通过contents属性和children属性都能获取直接子节点，但注意两者区别。
实例4：子孙节点
from bs4 import BeautifulSoup
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"><!--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>
"""
soup = BeautifulSoup(html,'lxml')
for i,content in enumerate(soup.p.descendants):
print(i,content)
print('type:',type(soup.p.descendants))结果：
实例5：父节点和祖先节点
from bs4 import BeautifulSoup
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"><!--Elsie--></a>,
and they lived at the bottom of a well.</p>
<p class = "story">...<p>
"""
soup = BeautifulSoup(html,'lxml')
print(list(enumerate(soup.p.parents)))
print(soup.p.parent)
print(type(soup.p.parents))
print(type(soup.p.children))
print(type(soup.p.contents))
print(type(soup.p.descendants))结果：
说明1：
parent属性获取的是父节点，parents属性获取的是所有的祖先节点。
说明2：
区分不同属性的类型。
实例6：兄弟选择器
from bs4 import BeautifulSoup
html ="""
<html><head><title>The Dormouse's story</title></head>
<body>
<p>hahaha</p>
Leraning
<a>C++</a>
HELLO
<a>Java</a>
World
<a>Python</a>
<a>JS</a>
<p class = "story">...<p>
"""
soup = BeautifulSoup(html,'lxml')
print('Next Sibling:',soup.a.next_sibling)
print('Prev Sibling:',soup.a.previous_sibling)
print('Next Siblings',list(enumerate(soup.a.next_siblings)))
print('Prev Siblings',list(enumerate(soup.a.previous_siblings)))结果：
实例7：方法选择器
find_all() API如下：find_all(name,attrs,recursive,text,**kwargs)
from bs4 import BeautifulSoup
html ="""
<div class = "C1">
<div class = "C2">
<h1>Hello</h1>
</div>
<div class = "C3">
<ul class = "U1" id = "list1">
<li class = "element">C++</li>
<li class = "element">Java</li>
</ul>
<ul class ="U2" id = "list2">
<li class = "element">Python></li>
</ul>
</div>
</div>
"""
from bs4 import BeautifulSoup
soup = BeautifulSoup(html,'lxml')
print(soup.find_all(name='ul')) #根据name查询元素
print()
print(soup.find_all(attrs={'class':'element'})) #根据attrs查询元素，参数类型是字典型
print()
print(soup.find_all(id='list2')) #另一种写法
print()
print(soup.find_all(class_='element')) #class是Python中关键字，这里记得加_结果：
补充：除了find_all()方法，还有find()方法，返回第一个匹配的元素。
实例8：CSS选择器
from bs4 import BeautifulSoup
html ="""
<div class = "C1">
<div class = "C2">
<h1>Hello</h1>
</div>
<div class = "C3">
<ul class = "U1" id = "list1">
<li class = "element">C++</li>
<li class = "element">Java</li>
</ul>
<ul class ="U2" id = "list2">
<li class = "element">Python</li>
</ul>
</div>
</div>
"""
from bs4 import BeautifulSoup
soup = BeautifulSoup(html,'lxml')
print(soup.select('.U1')) #选择器选择
print()
print(soup.select('ul li')[0]) #限定选择
print()
print(soup.select('ul li')[0].attrs['class']) #获取属性
print()
print(soup.select('li')[2].string) #获取文本
print()
print(soup.select('li')[2].get_text()) #另一个获取文本方法结果：
