BeautifulSoup4的安装
pip install beautifulsoup4
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BeautifulSoup4的使用
以下面一段html文档为例子，如：
html_doc = """
<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>
"""
常用方法
from bs4 import BeautifulSoup
soup = BeautifulSoup(html_doc, 'html.parser')
print(soup.prettify())
# 获取title标签
print(soup.title)
# <title>The Dormouse's story</title>
# 获取title标签名称
print(soup.title.name)
# title
# 获取title标签的内容
print(soup.title.string)
# The Dormouse's story
# 获取title的父标签
print(soup.title.parent)
# <head><title>The Dormouse's story</title></head>
# 获取title的父标签名称
print(soup.title.parent.name)
# head
# 获取p标签
print(soup.p)
# <p class="title"><b>The Dormouse's story</b></p>
# 获取p标签class属性
print(soup.p['class'])
# ['title'] #返回的是list
# 获取所有的a标签
print(soup.find_all('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>]
# 获取id='link3'的标签
print(soup.find(id="link3"))
# <a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>
# 获取所有的a标签的链接
for link in soup.find_all('a'):
print(link.get('href'))
# http://example.com/elsie
# http://example.com/lacie
# http://example.com/tillie
# 获取文档中所有文字内容
print(soup.get_text())
# The Dormouse's story
#
# The Dormouse's story
# Once upon a time there were three little sisters; and their names were
# Elsie,
# Lacie and
# Tillie;
# and they lived at the bottom of a well.
1. 对象的种类
BeautifulSoup将复杂HTML文档转换成一个复杂的属性结构，每个节点都是对象，所有对象分为4种类型：Tag，NavigabString，BeautifulSoup，Comment。
1.1 Tag：对象与XML或HTML原生文档中的tag相同
print(soup.title)
# <title>The Dormouse's story</title>
print(type(soup.title))
# <class 'bs4.element.Tag'>
Tag有2个重要的属性：name , attrs
name：tag的标签名称
print(soup.title.name)
# title
attrs：tag的属性
print(soup.p.attrs)
# {'class': ['title']}
1.2 NatigabString：标签的文本内容
print(soup.p.string)
# The Dormouse's story
print(type(soup.p.string))
# <class 'bs4.element.NavigableString'>
1.3 BeautifulSoup：表示一个文档内容，大部分时候，我们可以把它当做一个特殊的Tag。
print(soup.name)
# [document]
print(type(soup))
# <class 'bs4.BeautifulSoup'>
1.4 Comment：是一个特殊类型的 NavigableString 对象，其输出的内容不包括注释符号。
makeup='<p><!--Hello--></p>'
soup = BeatuifulSoup(makeup,'lxml')
print(soup.p.string)
# Hello
print(type(soup.p.string))
# <class 'bs4.element.Comment'>
2. 遍历文档树
2.1 子节点：.contents，.children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.p.contents)
# [<b>The Dormouse's story</b>] # 因为只有一个节点
print(soup.p.contents[0])
# [<b>The Dormouse's story</b>] 我们也可以获取列表的第一个标签。如果没有，会报错
tag的.children返回一个生成器，可以对tag的子节点进行循环。
print(type(soup.p.children))
# <class 'list_iterator'>
for child in soup.p.children:
print(child) # <b>The Dormouse's story</b>
2.2 所有子孙节点.descendants属性
.descendants属性可以对所有的tag子孙节点进行递归循环，和.childern类似。
for tag in soup.body.descendants:
print(tag)
# 输出结果：
# <b>The Dormouse's story</b>
# The Dormouse's story
3. 搜索文档树
3.1 find_all(name, attrs , recursive, text,limit, **kwargs)
find_all()参数：
name：查找名字为name的tag。（可以传入string，正则，列表）
attrs：tag的属性
recursive：是否递归，默认True
text：tag标签文本
limit：限制条数
3.1.1 name传入string
print(soup.find_all('p', attrs = {'class': 'title'}))
# [<p class="title"><b>The Dormouse's story</b></p>]
print(soup.find_all('p', text='...'))
# [<p class="story">...</p>]
print(soup.find_all('a', limit=2))
# [<a class="sister" href="http://example.com/elsie" id="link1">Elsie</a>, <a class="sister" href="http://example.com/lacie" id="link2">Lacie</a>]
3.1.2 name传入re正则表达式
for tag in soup.find_all(re.compile('^b')):
print(tag.name)
# body
# b
3.1.3 name传入列表
for tag in soup.find_all(['body','b']):
print(tag.name)
# body
# b
3.2 按CSS选择器搜索
3.2.1 通过标签名查找
print(soup.select('title'))
[<title>The Dormouse's story</title>]
3.2.2 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>]
3.2.3 通过id名查找
print(soup.select("#link1"))
# [<a class="sister" href="http://example.com/elsie" id="link1">Elsie</a>]
3.2.4 组合查找
print(soup.select('#link1,title'))
# [<title>The Dormouse's story</title>, <a class="sister" href="http://example.com/elsie" id="link1">Elsie</a>]
3.2.5 属性查找
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>]
3.2.6 获取内容
for tag in soup.select('a'):
print(tag.get_text())
# Elsie
# Lacie
# Tillie
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