Python网络爬虫与信息提取-Beautiful Soup 库入门2
一、BeautifulSoup类的基本元素
1、Tag的name（名字）
每个<tag>都有自己的名字，通过<tag>.name获取，字符串类型
>>> from bs4 import BeautifulSoup
>>> soup=BeautifulSoup(demo,"html.parser")
>>> soup.a.name
'a'
>>> soup.a.parent.name
'p'
>>> soup.a.parent.parent.name
'body'
2、Tag的attrs（属性）
一个<tag>可以有0个或多个属性，字典类型
>>> tag=soup.a
>>> tag.attrs
{'href': 'http://www.icourse163.org/course/BIT-268001', 'class': ['py1'], 'id': 'link1'}
>>> tag.attrs["class"]
['py1']
>>> tag.attrs['href']
'http://www.icourse163.org/course/BIT-268001'
>>> type(tag.attrs)
<class 'dict'>
>>> type(tag)
<class 'bs4.element.Tag'>
3、Tag的NavigableString
NavigableString可以跨越多个层次
>>> soup.a
<a class="py1" href="http://www.icourse163.org/course/BIT-268001" id="link1">Basic Python</a>
>>> soup.a.string
'Basic Python'
>>> type(soup.a.string)
<class 'bs4.element.NavigableString'>
4、Tag的Comment
Comment是一种特殊类型
>>> newsoup=BeautifulSoup("<b><!--This is a comment--></b><p>This is not a comment</p>",'html.parser')
>>> newsoup.b.string
'This is a comment'
>>> type(newsoup.b.string)
<class 'bs4.element.Comment'>
>>> newsoup.p.string
'This is not a comment'
>>> type(newsoup.p.string)
<class 'bs4.element.NavigableString'>
二、基于bs4库的HTML内容遍历方法
1、HTML基本格式
<>...</>构成了所属关系，形成了标签树的树形结构
2、标签树的下行遍历
| 属性 | 说明 |
| .contents | 子节点的列表，将<tag>所有儿子节点存入列表 |
| .children | 子节点的迭代类型，与.contents类似，用于循环遍历儿子节点 |
| .descendants | 子孙节点的迭代类型，包含所有子孙节点，用于循环遍历 |
BeautifulSoup类型是标签树的根节点
>>> soup=BeautifulSoup(demo,'html.parser')
>>> soup.head
<head><title>This is a python demo page</title></head>
>>> soup.head.contents
[<title>This is a python demo page</title>]
>>> soup.body.contents
['\n', <p class="title"><b>The demo python introduces several python courses.</b></p>, '\n', <p class="course">Python is a wonderful general-purpose programming language. You can learn Python from novice to professional by tracking the following courses:
<a class="py1" href="http://www.icourse163.org/course/BIT-268001" id="link1">Basic Python</a> and <a class="py2" href="http://www.icourse163.org/course/BIT-1001870001" id="link2">Advanced Python</a>.</p>, '\n']
>>> len(soup.body.contents)
5
>>> soup.body.contents[1]
<p class="title"><b>The demo python introduces several python courses.</b></p>
>>> for child in soup.body.children:
print(child) 遍历儿子节点
>>> for child in soup.body.descendants:
print(child) 遍历子孙节点
3、标签树的上行遍历
| 属性 | 说明 |
| .parent | 节点的父亲标签 |
| .parents | 节点先辈标签的迭代类型，用于循环遍历先辈节点 |
>>> soup=BeautifulSoup(demo,"html.parser")
>>> soup.title.parent
<head><title>This is a python demo page</title></head>
>>> soup.html.parent
<html><head><title>This is a python demo page</title></head>
<body>
<p class="title"><b>The demo python introduces several python courses.</b></p>
<p class="course">Python is a wonderful general-purpose programming language. You can learn Python from novice to professional by tracking the following courses:
<a class="py1" href="http://www.icourse163.org/course/BIT-268001" id="link1">Basic Python</a> and <a class="py2" href="http://www.icourse163.org/course/BIT-1001870001" id="link2">Advanced Python</a>.</p>
</body></html>
>>> soup.parent
>>>
>>> soup=BeautifulSoup(demo,"html.parser")
>>> for parent in soup.a.parents:
if parent is None:
print(parent)
else:
print(parent.name)
p
body
html
[document]
遍历所有先辈节点，包括soup本身，所以要区别判断
4、标签树的平行遍历
| 属性 | 说明 |
| .next_sibling | 返回按照HTML文本顺序的下一个平行节点标签 |
| .previous_sibling | 返回按照HTML文本顺序的上一个平行节点标签 |
| .next_siblings | 迭代类型，返回按照HTML文本顺序的后续所有平行节点标签 |
| .previous_siblings | 迭代类型，返回按照HTML文本顺序的前续所有平行节点标签 |
平行遍历发生在同一个父节点下的各节点间
>>> soup=BeautifulSoup(demo,"html.parser")
>>> soup.a.next_sibling
' and '
>>> soup.a.next_sibling.next_sibling
<a class="py2" href="http://www.icourse163.org/course/BIT-1001870001" id="link2">Advanced Python</a>
>>> soup.a.previous_sibling
'Python is a wonderful general-purpose programming language. You can learn Python from novice to professional by tracking the following courses:\r\n'
>>> soup.a.previous_sibling.previous_sibling
>>> soup.a.parent
<p class="course">Python is a wonderful general-purpose programming language. You can learn Python from novice to professional by tracking the following courses:
<a class="py1" href="http://www.icourse163.org/course/BIT-268001" id="link1">Basic Python</a> and <a class="py2" href="http://www.icourse163.org/course/BIT-1001870001" id="link2">Advanced Python</a>.</p>
>>> for sibling in soup.a.next_sibling:
print(sibling) 遍历后续节点
>>> for sibling in soup.a.previous_sibling:
print(sibling) 遍历前续节点
三、基于bs4库的HTML格式输出
1、bs4库的prettify()方法
>>> import requests
>>> r=requests.get("http://python123.io/ws/demo.html")
>>> demo=r.text
>>> from bs4 import BeautifulSoup
>>> soup=BeautifulSoup(demo,"html.parser")
>>> print(soup.prettify())
<html>
<head>
<title>
This is a python demo page
</title>
</head>
<body>
<p class="title">
<b>
The demo python introduces several python courses.
</b>
</p>
<p class="course">
Python is a wonderful general-purpose programming language. You can learn Python from novice to professional by tracking the following courses:
<a class="py1" href="http://www.icourse163.org/course/BIT-268001" id="link1">
Basic Python
</a>
and
<a class="py2" href="http://www.icourse163.org/course/BIT-1001870001" id="link2">
Advanced Python
</a>
</p>
</body>
</html>
.prettify()为HTML文本<>及其内容增加更加'\n'
.prettify()可用于标签，方法：<tag>.prettify()
>>> print(soup.a.prettify())
<a class="py1" href="http://www.icourse163.org/course/BIT-268001" id="link1">
Basic Python
</a>
2、bs4库的编码
bs4库将任何HTML输入都变成utf-8编码
Python 3.x默认支持编码为utf-8，解析无障碍
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