Comprehensive understanding of Python crawler xlml parsing library

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1.Xpath

Xpath is a language for finding information in XML, which can be used to Traverse elements and attributes in XML documents. Both XQuery and xpoint are built on xpath expression

2. Node

Parent (parent), child (children), brother ( sibling), ancestor (ancetstor), descendant (Decendant)

3. Select node

Path expression

##@ Select attributes//@langSelect all attributes named lang

Expression	Description	Path expression	Result
nodename	Select all child nodes on this node	bookstore	Select all child nodes of the bookstore element
/	Select	/bookstore from the root node	Select the root element bookstore, which is the absolute path
//	Select nodes in the document from the current node matching the selection, regardless of position	//book	Select all book child elements regardless of their position in the document
.	Select the current node	bookstore//book	Select all book elements in the bookstore descendants
..	Select the parent node of the current node

Predicate

The predicate is used to

find a specific node or a node containing a specified value

The predicate is embedded in square brackets

Path expressionResult/bookstore/book[1]Select the bookstore child The first book element of the element/bookstore/book[last()]Select the last book element that belongs to the bookstore child element/bookstore/book[last()-1]Select the penultimate book element that belongs to the bookstore child element/bookstore/book [position()72f92fdc62658ddba2b9a39df3b8fb6735.0]##Select unknown nodes (wildcards)

Select all book elements of the bookstore element, and the price value is greater than 35.0

* Match any element node

@* Match any attribute node

node() Match any type of node

4.lxml usage

#!/usr/bin/python
#_*_coding:utf-8_*_

from lxml import etree

text='''
<p>
 <ul>
  <li class="item-0"><a href="link1.html" rel="external nofollow" rel="external nofollow" >first item</a></li>
  <li class="item-1"><a href="link2.html" rel="external nofollow" >second item</a></li>
  <li class="item-inactive"><a href="link3.html" rel="external nofollow" >third item</a></li>
  <li class="item-1"><a href="link4.html" rel="external nofollow" >fourth item</a></li>
  <li class="item-0"><a href="link5.html" rel="external nofollow" >fifth item</a>
 </ul>
</p>
  &#39;&#39;&#39;

# html=etree.HTML(text) #html对象，存储在地址中，有自动修正功能
# result=etree.tostring(html) #将html对象转化为字符串

html=etree.parse(&#39;hello.html&#39;)
# result=etree.tostring(html,pretty_print=True)
# print result
print type(html)
result= html.xpath(&#39;//li&#39;)
print result
print len(result)
print type(result)
print type(result[0])
print html.xpath(&#39;//li/@class&#39;) # 获取li标签下的所有的class
print html.xpath(&#39;//li/a[@href="link1.html" rel="external nofollow" rel="external nofollow" ]&#39;) #获取li标签下href为link1的<a>标签
print html.xpath(&#39;//li//span&#39;) #获取li标签下所有的span标签
print html.xpath(&#39;//li[last()-1]/a&#39;)[0].text #获取倒数第二个元素的内容

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