Python XML Processing


Contents

XML is a moveable, open source language that permits programmers to expand applications that know how to be interpreted by other applications, in spite of of operating system and/or developmental language.

What is XML?

XML stands for Extensible Markup Language. This language is considered similar to HTML or SGML. This is optional for accessing as an open normal by the World Wide Web Consortium.

XML is extremely obliging for keeping track of little to medium amounts of data with no requiring a SQL-based backbone.

XML Parser Architectures and APIs

The Python normal library supplies a negligible but functional set of interfaces to work with XML.

The two mainly essential and generally employed APIs to XML data are the SAX and DOM interfaces.

  • Easy API for XML (SAX) − Here, you list recalls for occasions of attention and then let the parser carry on through the document. This is functional when your files are large or you have reminiscence limitations, it parses the folder as it understands it from disk and the complete file is never stored in memory.
  • Document Object Model (DOM) API − This is a World Wide Web Consortium advice wherein the whole file is interpret into memory and amassed in a hierarchical (tree-based) form to stand for all the features of an XML text.

SAX clearly never process in order as fast as DOM can while operational with large files. On the other give, by DOM wholly can actually kill your reserves, particularly if used on a lot of small files.

SAX is read-only, as DOM permits changes to the XML file. As these two diverse APIs factually harmonize each other, there is no motive both for large projects.

For all our XML code examples, let's use a simple XML file movies.xml as an input:

<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>

Parsing XML with SAX APIs

SAX is an average edge for event-driven XML parsing. Parsing XML with SAX normally necessitates you to generate your own ContentHandler by subclassing xml.sax.ContentHandler.

Your ContentHandler handles the particular tags and attributes of your flavor(s) of XML. A ContentHandler object provides methods to handle various parsing events. Its owning parser calls ContentHandler methods as it parses the XML file.

The methods startDocument and endDocument are called at the start and the end of the XML file. The method characters(text) is passed character data of the XML file via the parameter text.

The ContentHandler is called at the start and end of each element. If the parser is not in namespace mode, the methods startElement(tag, attributes) and endElement(tag) are called; otherwise, the corresponding methods startElementNS and endElementNS are called. Here, tag is the element tag, and attributes is an Attributes object.

Here are other important methods to understand before proceeding −

The make_parser Method

Following method creates a new parser object and returns it. The parser object created will be of the first parser type the system finds.

xml.sax.make_parser( [parser_list] )

Here is the detail of the parameters:

  • parser_list − The optional argument consisting of a list of parsers to use which must all implement the make_parser method.

The parse Method

Following method creates a SAX parser and uses it to parse a document.

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

Here is the detail of the parameters −

  • xmlfile − This is the name of the XML file to read from.
  • contenthandler − This must be a ContentHandler object.
  • errorhandler − If specified, errorhandler must be a SAX ErrorHandler object.

The parseString Method

There is one more method to create a SAX parser and to parse the specified XML string.

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

Here is the detail of the parameters −

  • xmlstring − This is the name of the XML string to read from.
  • contenthandler − This must be a ContentHandler object.
  • errorhandler − If specified, errorhandler must be a SAX ErrorHandler object.

Example

#!/usr/bin/python

import xml.sax

class MovieHandler( xml.sax.ContentHandler ):
   def __init__(self):
      self.CurrentData = ""
      self.type = ""
      self.format = ""
      self.year = ""
      self.rating = ""
      self.stars = ""
      self.description = ""

   # Call when an element starts
   def startElement(self, tag, attributes):
      self.CurrentData = tag
      if tag == "movie":
         print "*****Movie*****"
         title = attributes["title"]
         print "Title:", title

   # Call when an elements ends
   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 = ""

   # Call when a character is read
   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__"):
   
   # create an XMLReader
   parser = xml.sax.make_parser()
   # turn off namepsaces
   parser.setFeature(xml.sax.handler.feature_namespaces, 0)

   # override the default ContextHandler
   Handler = MovieHandler()
   parser.setContentHandler( Handler )
   
   parser.parse("movies.xml")

This would produce following result:

*****Movie*****
Title: Enemy Behind
Type: War, Thriller
Format: DVD
Year: 2003
Rating: PG
Stars: 10
Description: Talk about a US-Japan war
*****Movie*****
Title: Transformers
Type: Anime, Science Fiction
Format: DVD
Year: 1989
Rating: R
Stars: 8
Description: A schientific fiction
*****Movie*****
Title: Trigun
Type: Anime, Action
Format: DVD
Rating: PG
Stars: 10
Description: Vash the Stampede!
*****Movie*****
Title: Ishtar
Type: Comedy
Format: VHS
Rating: PG
Stars: 2
Description: Viewable boredom

For a complete detail on SAX API documentation, please refer to standard Python SAX APIs.

Parsing XML with DOM APIs

The Document Object Model ("DOM") is a cross-language API from the World Wide Web Consortium (W3C) for accessing and modifying XML documents.

The DOM is tremendously practical for random-access applications. SAX only tolerates you a vision of one bit of the document at an instance. If you are searching at one SAX element, you encompass no contact to another.

Here is the easiest method to load an XML document and to make a minidom object by the xml.dom module. The minidom object gives a easy parser method that rapidly creates a DOM tree from the XML file.

The sample phrase calls the parse( file [,parser] ) function of the minidom object to parse the XML file designated by file into a DOM tree object.

#!/usr/bin/python

from xml.dom.minidom import parse
import xml.dom.minidom

# Open XML document using minidom parser
DOMTree = xml.dom.minidom.parse("movies.xml")
collection = DOMTree.documentElement
if collection.hasAttribute("shelf"):
   print "Root element : %s" % collection.getAttribute("shelf")

# Get all the movies in the collection
movies = collection.getElementsByTagName("movie")

# Print detail of each movie.
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

This would produce the following result:

Root element : New Arrivals
*****Movie*****
Title: Enemy Behind
Type: War, Thriller
Format: DVD
Rating: PG
Description: Talk about a US-Japan war
*****Movie*****
Title: Transformers
Type: Anime, Science Fiction
Format: DVD
Rating: R
Description: A schientific fiction
*****Movie*****
Title: Trigun
Type: Anime, Action
Format: DVD
Rating: PG
Description: Vash the Stampede!
*****Movie*****
Title: Ishtar
Type: Comedy
Format: VHS
Rating: PG
Description: Viewable boredom

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