Complete Guide For Python Programming (2015)

PYTHON XML PROCESSING

What is XML?

XML is, Extensible Markup Language (XML) and its like HTML or SGML. XML is a portable, open source language that allows the programmers to develop applications that can be read by other applications, regardless of operating system and/or developmental language. XML is extremely useful for keeping track of small to medium amounts of data.

XML Parser Architectures and APIs:

The Python standard library provides a set of interfaces to work with XML. The two most basic and broadly used APIs to XML data are the SAX and DOM interfaces.

• Simple API for XML (SAX): Here, you register callbacks for events of interest and then let the parser proceed through the document. This is useful when your documents are large or you have memory limitations, it parses the file as it reads it from disk and the entire file is never stored in memory.

• Document Object Model (DOM) API: This is a World Wide Web Consortium recommendation wherein the entire file is read into memory and stored in a hierarchical (tree-based) form to represent all the features of an XML document.

The thing is that SAX can't process information as fast as DOM, when working with large files. On the other hand, using DOM can kill your resources, especially if used on a lot of small files. SAX is read-only, while DOM allows changes to the XML file. As these two APIs complement each other, there is no reason why you can't use them both for large projects. Let’s see a simple example for XML file movies.xml:

<type>War, Thriller</type>

<description>Talk about a US-Japan war</description>

</movie>

<type>Anime, Science Fiction</type>

<description>A schientific fiction</description>

</movie>

<type>Anime, Action</type>

<description>Vash the Stampede!</description>

</movie>

<type>Comedy</type>

<description>Viewable boredom</description>

</movie>

</collection>

Parsing XML with SAX APIs:

SAX is a standard interface for event-driven XML parsing. For Parsing XML with SAX, you need to create your own ContentHandler by subclassing xml.sax.ContentHandler. Your ContentHandler handles the particular tags and attributes of your flavor of XML. A ContentHandler object provides methods to handle various parsing events. Its owning parser calls ContentHandler methods as it passes the XML file. The methods startDocument and endDocument are called at the start and the end of the XML file. The ContentHandler is called at the start and end of each element. Here are some methods to understand before proceeding:

The make_parser Method:

This 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 parameter 'parser_list', is the optional argument consisting of a list of parsers to use which must all implement the make_parser method.

The parse Method:

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

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

Here parameters 'xmlfile', is the name of the XML file to read from. 'contenthandler', must be a ContentHandler object. and '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 parameters 'xmlstring', is the name of the XML string to read from. 'contenthandler', must be a ContentHandler object. 'errorhandler', must be a SAX ErrorHandler object.

For 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")

Output:

*****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

Parsing XML with DOM APIs:

The Document Object Model or "DOM," is a cross-language API which is used for accessing and modifying the XML documents. The DOM is extremely useful for random-access applications. SAX allows to use one document at a time. If you are looking at one SAX element, you have no access to another one. the easiest way to quickly load an XML document and to create a minidom object is by using the xml.dom module. The minidom object provides a simple parser method that will quickly create a DOM tree from the XML file.

For Example:

#!/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

Output:

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