There are a variety of XSLT processors available on the market, both free and commercial. The power of XSLT is in the transformations that the language allows, but the actual work is completed by the processor. Depending on your environment, you may choose a processor based on speed or on accessibility from a particular platform such as Python. Alternatively, you may choose a processor that you can drive programmatically.

The XSLT processor’s job is to take an XSL stylesheet and perform its transformation rules against an existing XML document to produce a new transformed document. The W3C states that XSLT is for transforming XML to XML, which is true, but it can be used to generate HTML or other formats as well. It is frequently used to transform XML to HTML or XHTML for viewing in a web browser.

XSLT is a language unto itself, and has nothing in particular to do with Python. As such, you can convert documents for use in your Python programs with any XSLT processor. However, if you are hoping to embed XSLT functionality within your Python programs, you need a processor accessible from Python either natively (such as 4XSLT) or by a bridge mechanism (such as using MSXML3.0 from Python, as covered in Appendix E).

For Python, the 4XSLT package is an open source XSLT processor that can be driven from the command line as well as embedded in your Python programs—it is primarily implemented in Python, but includes some modules written in C for improved performance. 4XSLT is available from Fourthought, Inc. as part of the 4Suite package (see http://www.4suite.org/).

Other XSLT processors exist for other languages and platforms, but can still batch process transformations for use in your Python applications. Microsoft’s Internet Explorer has an XSLT processor embedded within it, and can transform an XML document into HTML in the client’s browser (though versions prior to 6.0 are horribly noncompliant). SAXON is a collection of XML tools, including a Java-based XSLT processor capable of running in any Java virtual machine. Sablotron is a fast C++ XSLT processor. The W3C’s XSLT site (http://www.w3.org/Style/XSL/) contains numerous links to XSLT processing software.

For the remainder of this chapter, we use the 4XSLT processor as it’s completely Python-based, and its functionality is accessible at runtime from your Python applications.