bs4作为python解析html与xml的一枚神器，很有必要简明教程一下。

如何安装

在Debian和Ubuntu下可以采用如下方式：
$ apt-get install python-bs4

也可以通过PyPi的安装方式：
$ easy_install beautifulsoup4
$ pip install beautifulsoup4

当然，还可以通过下载源码，用setup.py的方式进行安装：
$ python setup.py install

简明使用

下面先举个例子：

from bs4 import BeautifulSoup

html_doc = """


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.

...
"""

soup = BeautifulSoup(html_doc)
print(soup.prettify())

# 我们可以看到html代码已经被格式化了
# 
#  
#   
#  
#  
#   

#    
#     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.
#   
#   

#    ...
#   
#  
# 

# 下面我们再看看如何获得html的各个node

soup.title
# 

soup.title.name
# u'title'

soup.title.string
# u'The Dormouse's story'

soup.title.parent.name
# u'head'

soup.p
# 
The Dormouse's story

soup.p['class']
# u'title'

soup.a
# Elsie

soup.find_all('a')
# [Elsie,
#  Lacie,
#  Tillie]

soup.find(id="link3")
# Tillie

# 如果我们想提取所有的锚链接，那实在是太简单了

for link in soup.find_all('a'):
    print(link.get('href'))
# http://example.com/elsie
# http://example.com/lacie
# http://example.com/tillie

# 如果我们只想提取所有的文字呢？那也还是很简单

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.
#
# ...

进阶使用

##Tag

soup = BeautifulSoup('Extremely bold')
tag = soup.b
type(tag)
# 

##Name
tag.name
# u'b'

tag.name = "blockquote"
tag
# 
Extremely bold

##Attributes
tag['class']
# u'boldest'

tag.attrs
# {u'class': u'boldest'}
tag['class'] = 'verybold'
tag['id'] = 1
tag
# 
Extremely bold

del tag['class']
del tag['id']
tag
# 
Extremely bold

tag['class']
# KeyError: 'class'
print(tag.get('class'))
# None

##Multi-valued Attributes
css_soup = BeautifulSoup('
')
css_soup.p['class']
# ["body", "strikeout"]

css_soup = BeautifulSoup('
')
css_soup.p['class']
# ["body"]

id_soup = BeautifulSoup('
')
id_soup.p['id']
# 'my id'

rel_soup = BeautifulSoup('
Back to the homepage
')
rel_soup.a['rel']
# ['index']
rel_soup.a['rel'] = ['index', 'contents']
print(rel_soup.p)
# 
Back to the homepage


xml_soup = BeautifulSoup('
', 'xml')
xml_soup.p['class']
# u'body strikeout'

##comments

markup = ""
soup = BeautifulSoup(markup)
comment = soup.b.string
type(comment)
# 
comment
# u'Hey, buddy. Want to buy a used parser'
print(soup.b.prettify())
# 
#  
# 

from bs4 import CData
cdata = CData("A CDATA block")
comment.replace_with(cdata)

print(soup.b.prettify())
# 
#