Detailed explanation of XML parsing in Python

What is XML?

XML refers to Extensible Markup Language (eXtensible Markup Language).

XML is designed to transmit and store data.

XML is a set of rules that define semantic tags that divide a document into parts and identify these parts.

It is also a meta-markup language, that is, it defines a syntactic language for defining other semantic and structured markup languages ​​related to specific fields.

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pythonParsing of XML

Common XMLProgrammingInterfaceThere are DOM and SAX, these two The two interfaces process XML files in different ways, and of course the usage scenarios are also different.

Python has three methods to parse XML, SAX, DOM, and ElementTree:

1.SAX (simple API for XML )

The python standard library contains a SAX parser. SAX uses events to drive the model by triggering events one by one and calling the user during the process of parsing XML. Defined Callback function to process XML files.

2.DOM(Document Object Model)

Parse the XML data into a tree in memory, through Manipulate XML using tree operations.

3.ElementTree (Element Tree)

ElementTree is like a lightweight DOM with a convenient and friendly API. The code has good usability, is fast, and consumes less memory.

Note: Because DOM needs to map XML data to a tree in memory, firstly, it is relatively slow, and secondly, it consumes more memory, while SAX streaming reading of XML files is faster. It takes up less memory, but requires the user to implement a callback function (handler).

The content of the XML example file movies.xml used in this chapter is as follows:

<collection shelf="New Arrivals"><movie title="Enemy Behind">
   <type>War, Thriller</type>
   <format>DVD</format>
   <year>2003</year>
   <rating>PG</rating>
   <stars>10</stars>
   <description>Talk about a US-Japan war</description></movie><movie title="Transformers">
   <type>Anime, Science Fiction</type>
   <format>DVD</format>
   <year>1989</year>
   <rating>R</rating>
   <stars>8</stars>
   <description>A schientific fiction</description></movie>
   <movie title="Trigun">
   <type>Anime, Action</type>
   <format>DVD</format>
   <episodes>4</episodes>
   <rating>PG</rating>
   <stars>10</stars>
   <description>Vash the Stampede!</description></movie><movie title="Ishtar">
   <type>Comedy</type>
   <format>VHS</format>
   <rating>PG</rating>
   <stars>2</stars>
   <description>Viewable boredom</description></movie></collection>

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python uses SAX to parse xml

SAX is an event-driven API .

Using SAX to parse XML documents involves two parts: the parser and the event handler.

The parser is responsible for reading the XML document and sending events to the event processor, such as element start and element end events;

The event processor is responsible for responding to the event and passing the XML data is processed.

• 1. Process large files;

• 2. Only need part of the file, or only need specific information from the file .

• 3. When you want to create your own object model.

To use sax to process xml in python, you must first introduce the parse function in xml.sax and the ContentHandler in xml.sax.handler.

ContentHandler class method introduction

characters(content) method

Calling timing:

Start from the line, before encountering the label , there are characters, and the value of content is these strings .

From one label, there are characters before encountering the next label, and the value of content is these strings.

From a label, there are characters before encountering the line terminator, and the value of content is these strings. The

tag can be a start tag or an end tag.

startDocument() method

Called when the document is started.

endDocument() method

Called when the parser reaches the end of the document.

startElement(name, attrs) method

Called when an XML start tag is encountered, name is the name of the tag, attrs is the attribute of the tag Dictionary of values.

endElement(name) method

Called when an XML end tag is encountered.

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make_parser method

The following method creates a new parser object and returns it.

xml.sax.make_parser( [parser_list] )

Parameter description:

• parser_list - Optional parameter, parser list

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parser method

The following method creates a SAX parser and parses the xml document:

xml.sax.parse( xmlfile, contenthandler[, errorhandler])

Parameter description:

• xmlfile - xml file name

• contenthandler - must be a ContentHandler object

• errorhandler - 如果指定该参数，errorhandler必须是一个SAX ErrorHandler对象

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parseString方法

parseString方法创建一个XML解析器并解析xml字符串：

xml.sax.parseString(xmlstring, contenthandler[, errorhandler])

参数说明:

• xmlstring - xml字符串

• contenthandler - 必须是一个ContentHandler的对象

• errorhandler - 如果指定该参数，errorhandler必须是一个SAX ErrorHandler对象

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Python 解析XML实例

#!/usr/bin/python# -*- coding: UTF-8 -*-import xml.saxclass MovieHandler( xml.sax.ContentHandler ):
   def init(self):
      self.CurrentData = ""
      self.type = ""
      self.format = ""
      self.year = ""
      self.rating = ""
      self.stars = ""
      self.description = ""

   # 元素开始事件处理
   def startElement(self, tag, attributes):
      self.CurrentData = tag      if tag == "movie":
         print "*****Movie*****"
         title = attributes["title"]
         print "Title:", title   # 元素结束事件处理
   def endElement(self, tag):
      if self.CurrentData == "type":
         print "Type:", self.type      elif self.CurrentData == "format":
         print "Format:", self.format      elif self.CurrentData == "year":
         print "Year:", self.year      elif self.CurrentData == "rating":
         print "Rating:", self.rating
      elif self.CurrentData == "stars":
         print "Stars:", self.stars
      elif self.CurrentData == "description":
         print "Description:", self.description
      self.CurrentData = ""

   # 内容事件处理
   def characters(self, content):
      if self.CurrentData == "type":
         self.type = content
      elif self.CurrentData == "format":
         self.format = content
      elif self.CurrentData == "year":
         self.year = content
      elif self.CurrentData == "rating":
         self.rating = content
      elif self.CurrentData == "stars":
         self.stars = content
      elif self.CurrentData == "description":
         self.description = content
  
if ( name == "main"):
   
   # 创建一个 XMLReader
   parser = xml.sax.make_parser()
   # turn off namepsaces
   parser.setFeature(xml.sax.handler.feature_namespaces, 0)

   # 重写 ContextHandler
   Handler = MovieHandler()
   parser.setContentHandler( Handler )
   
   parser.parse("movies.xml")

以上代码执行结果如下：

*****Movie*****Title: Enemy BehindType: War, ThrillerFormat: DVDYear: 2003Rating: PGStars: 10Description: Talk about a US-Japan war*****Movie*****Title: TransformersType: Anime, Science FictionFormat: DVDYear: 1989Rating: RStars: 8Description: A schientific fiction*****Movie*****Title: TrigunType: Anime, ActionFormat: DVDRating: PGStars: 10Description: Vash the Stampede!*****Movie*****Title: IshtarType: ComedyFormat: VHSRating: PGStars: 2Description: Viewable boredom

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使用xml.dom解析xml

文件对象模型（Document Object Model，简称DOM），是W3C组织推荐的处理可扩展置标语言的标准编程接口。

一个 DOM 的解析器在解析一个 XML 文档时，一次性读取整个文档，把文档中所有元素保存在内存中的一个树结构里，之后你可以利用DOM 提供的不同的函数来读取或修改文档的内容和结构，也可以把修改过的内容写入xml文件。

python中用xml.dom.minidom来解析xml文件，实例如下：

#!/usr/bin/python# -*- coding: UTF-8 -*-from xml.dom.minidom import parseimport xml.dom.minidom# 使用minidom解析器打开 XML 文档DOMTree = xml.dom.minidom.parse("movies.xml")collection = DOMTree.documentElementif collection.hasAttribute("shelf"):
   print "Root element : %s" % collection.getAttribute("shelf")# 在集合中获取所有电影movies = collection.getElementsByTagName("movie")# 打印每部电影的详细信息for movie in movies:
   print "*****Movie*****"
   if movie.hasAttribute("title"):
      print "Title: %s" % movie.getAttribute("title")

   type = movie.getElementsByTagName('type')[0]
   print "Type: %s" % type.childNodes[0].data
   format = movie.getElementsByTagName('format')[0]
   print "Format: %s" % format.childNodes[0].data
   rating = movie.getElementsByTagName('rating')[0]
   print "Rating: %s" % rating.childNodes[0].data
   description = movie.getElementsByTagName('description')[0]
   print "Description: %s" % description.childNodes[0].data

以上程序执行结果如下：

Root element : New Arrivals*****Movie*****Title: Enemy BehindType: War, ThrillerFormat: DVDRating: PGDescription: Talk about a US-Japan war*****Movie*****Title: TransformersType: Anime, Science FictionFormat: DVDRating: RDescription: A schientific fiction*****Movie*****Title: TrigunType: Anime, ActionFormat: DVDRating: PGDescription: Vash the Stampede!*****Movie*****Title: IshtarType: ComedyFormat: VHSRating: PGDescription: Viewable boredom

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