XML is a portable, open source language that allows programmers to develop applications that can be read by other applications, regardless of operating system and/or developmental language.
What is XML?
The Extensible Markup Language (XML) is a markup language much like HTML or SGML. This is recommended by the World Wide Web Consortium and available as an open standard.
XML is extremely useful for keeping track of small to medium amounts of data without requiring a SQL-based backbone.
XML Parser Architectures and APIs
The Python standard library provides a minimal but useful 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.
SAX obviously cannot process information as fast as DOM can when working with large files. On the other hand, using DOM exclusively can really kill your resources, especially if used on a lot of small files.
SAX is read-only, while DOM allows changes to the XML file. Since these two different APIs literally complement each other, there is no reason why you cannot use them 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 a standard interface for event-driven XML parsing. Parsing XML with SAX generally requires you to create 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 extremely useful for random-access applications. SAX only allows you a view of one bit of the document at a time. If you are looking at one SAX element, you have no access to another.
Here is the easiest way to quickly load an XML document and to create a minidom object using the xml.dom module. The minidom object provides a simple parser method that quickly 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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