BeautifulSoup 可以将lxml作为默认的解析器使用，同样lxml可以单独使用。下面比较这两者之间优缺点：
BeautifulSoup和lxml原理不一样，BeautifulSoup是基于DOM的，会载入整个文档，解析整个DOM树，因此时间和内存开销都会比较大很多。而lxml是使用XPath技术查询和处理HTML/XML文档的库，只会局部遍历，所以速度会快一些。幸好现在BeautifulSoup可以使用lxml作为默认解析库
关于XPath的用法，请点击：https://www.cnblogs.com/guguobao/p/9401643.html
示例：
#coding:utf-8
from lxml import etree
html_str = """
<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>
"""
html = etree.HTML(html_str)
result = etree.tostring(html)
print(result)
可以发现html_str最后是没有闭合的，但可以通过etree.tostring(html)自动修正HTML代码
from lxml import etree
html = etree.parse('index.html')
result = etree.tostring(html, pretty_print=True)
print(result)
除了读取字符串之外，lxml还可以直接读取html文件。假设html_str被复制index.html,则可以用parse方法解析(代码在上)。
接下来使用XPath语句抽取html中的URL
html = etree.HTML(html_str)
urls = html.xpath(".//*[@class='sister']/@href")
print urls