Chapter 23. Structured Text: XML

XML, the eXtensible Markup Language, has taken the programming world by storm over the last few years. Like SGML, XML is a metalanguage, a language to describe markup languages. On top of the XML 1.0 specification, the XML community (in good part inside the World Wide Web Consortium, W3C) has standardized other technologies, such as various schema languages, Namespaces, XPath, XLink, XPointer, and XSLT.

Industry consortia in many fields have defined industry-specific markup languages on top of XML, to facilitate data exchange among applications in the various fields. Such industry standards let applications exchange data even if the applications are coded in different languages and deployed on different platforms by different firms. XML, related technologies, and XML-based markup languages are the basis of interapplication, cross-language, cross-platform data interchange in modern applications.

Python has excellent support for XML. The standard Python library supplies the xml package, which lets you use fundamental XML technology quite simply. The third-party package PyXML (available at http://pyxml.sf.net) extends the standard library's xml with validating parsers, richer DOM implementations, and advanced technologies such as XPath and XSLT. Downloading and installing PyXML upgrades Python's own xml packages, so it can be a good idea to do so even if you don't use PyXML-specific features.

On top of PyXML, you can choose to install yet another freely available third-party package, 4Suite (available at http://4suite.org). 4Suite provides yet more XML parsers for special niches, advanced technologies such as XLink and XPointer, and code supporting standards built on top of XML, such as the Resource Description Framework (RDF).

As an alternative to Python's built-in XML support, PyXML, and 4Suite, you can try ReportLab's new pyRXP, a fast validating XML parser based on Tobin's RXP. pyRXP is DOM-like in that it constructs an in-memory representation of the whole XML document you're parsing. However, pyRXP does not construct a DOM-compliant tree, but rather a lightweight tree of Python tuples to save memory and enhance speed. For more information on pyRXP, see http://www.reportlab.com/xml/pyrxp.html.

For coverage of all aspects of XML and of how you can process XML with Python, I recommend Python & XML, by Christopher Jones and Fred Drake (O'Reilly). In this chapter, I cover only the essentials of the standard library's xml package, taking some elementary knowledge of XML itself for granted.