Python and bindings

# The XSLT C library for GNOME

## Python and bindings

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There is a number of language bindings and wrappers available for libxml2, the list below is not exhaustive. Please contact the xml-bindings@gnome.org (archives) in order to get updates to this list or to discuss the specific topic of libxml2 or libxslt wrappers or bindings:

- Matt Sergeant developped XML::LibXML and XML::LibXSLT, Perl wrappers for libxml2/libxslt as part of the AxKit XML application server
- Dave Kuhlman provides and earlier version of the libxml/libxslt wrappers for Python
- Petr Kozelka provides Pascal units to glue libxml2 with Kylix, Delphi and other Pascal compilers
- Wai-Sun "Squidster" Chia provides bindings for Ruby and libxml2 bindings are also available in Ruby through the libgdome-ruby module maintained by Tobias Peters.
- Steve Ball and contributors maintains libxml2 and libxslt bindings for Tcl
- Peter Jones maintains C++ bindings for libxslt within xmlwrapp
- Mike Phillips provides a module using libxslt for PHP.
- LibxmlJ is an effort to create a 100% JAXP-compatible Java wrapper for libxml2 and libxslt as part of GNU ClasspathX project.
- Patrick McPhee provides Rexx bindings fof libxml2 and libxslt, look for RexxXML.
- Satimage provides XMLLib osax. This is an osax for Mac OS X with a set of commands to implement in AppleScript the XML DOM, XPATH and XSLT.

The libxslt Python module depends on the libxml2 Python module.

The distribution includes a set of Python bindings, which are guaranteed to be maintained as part of the library in the future, though the Python interface have not yet reached the completeness of the C API.

Stéphane Bidoul maintains a Windows port of the Python bindings.

Note to people interested in building bindings, the API is formalized as an XML API description file which allows to automate a large part of the Python bindings, this includes function descriptions, enums, structures, typedefs, etc... The Python script used to build the bindings is python/generator.py in the source distribution.

To install the Python bindings there are 2 options:

- If you use an RPM based distribution, simply install the libxml2-python RPM and the libxslt-python RPM.
- Otherwise use the libxml2-python module distribution corresponding to your installed version of libxml2 and libxslt. Note that to install it you will need both libxml2 and libxslt installed and run "python setup.py build install" in the module tree.

The distribution includes a set of examples and regression tests for the python bindings in the `python/tests` directory. Here are some excepts from those tests:

### basic.py:

This is a basic test of XSLT interfaces: loading a stylesheet and a document, transforming the document and saving the result.

    import libxml2
    import libxslt

    styledoc = libxml2.parseFile("test.xsl")
    style = libxslt.parseStylesheetDoc(styledoc)
    doc = libxml2.parseFile("test.xml")
    result = style.applyStylesheet(doc, None)
    style.saveResultToFilename("foo", result, 0)
    style.freeStylesheet()
    doc.freeDoc()
    result.freeDoc()

The Python module is called libxslt, you will also need the libxml2 module for the operations on XML trees. Let's have a look at the objects manipulated in that example and how is the processing done:

- `styledoc` : is a libxml2 document tree. It is obtained by parsing the XML file "test.xsl" containing the stylesheet.
- `style` : this is a precompiled stylesheet ready to be used by the following transformations (note the plural form, multiple transformations can resuse the same stylesheet).
- `doc` : this is the document to apply the transformation to. In this case it is simply generated by parsing it from a file but any other processing is possible as long as one get a libxml2 Doc. Note that HTML tree are suitable for XSLT processing in libxslt. This is actually how this page is generated !
- `result` : this is a document generated by applying the stylesheet to the document. Note that some of the stylesheet information may be related to the serialization of that document and as in this example a specific saveResultToFilename() method of the stylesheet should be used to save it to a file (in that case to "foo").

Also note the need to explicitely deallocate documents with freeDoc() except for the stylesheet document which is freed when its compiled form is garbage collected.

### extfunc.py:

This one is a far more complex test. It shows how to modify the behaviour of an XSLT transformation by passing parameters and how to extend the XSLT engine with functions defined in python:

    import libxml2
    import libxslt
    import string

    nodeName = None
    def f(ctx, str):
        global nodeName

        #
        # Small check to verify the context is correcly accessed
        #
        try:
            pctxt = libxslt.xpathParserContext(_obj=ctx)
            ctxt = pctxt.context()
            tctxt = ctxt.transformContext()
            nodeName = tctxt.insertNode().name
        except:
            pass

        return string.upper(str)

    libxslt.registerExtModuleFunction("foo", "http://example.com/foo", f)

This code defines and register an extension function. Note that the function can be bound to any name (foo) and how the binding is also associated to a namespace name "http://example.com/foo". From an XSLT point of view the function just returns an upper case version of the string passed as a parameter. But the first part of the function also read some contextual information from the current XSLT processing environement, in that case it looks for the current insertion node in the resulting output (either the resulting document or the Result Value Tree being generated), and saves it to a global variable for checking that the access actually worked.

For more information on the xpathParserContext and transformContext objects check the libray internals description. The pctxt is actually an object from a class derived from the libxml2.xpathParserContext() with just a couple more properties including the possibility to look up the XSLT transformation context from the XPath context.

    styledoc = libxml2.parseDoc("""
    <xsl:stylesheet version='1.0'
      xmlns:xsl='http://www.w3.org/1999/XSL/Transform'
      xmlns:foo='http://example.com/foo'
      xsl:exclude-result-prefixes='foo'>

      <xsl:param name='bar'>failure</xsl:param>
      <xsl:template match='/'>
        <article><xsl:value-of select='foo:foo($bar)'/></article>
      </xsl:template>
    </xsl:stylesheet>
    """)

Here is a simple example of how to read an XML document from a python string with libxml2. Note how this stylesheet:

- Uses a global parameter `bar`
- Reference the extension function f
- how the Namespace name "http://example.com/foo" has to be bound to a prefix
- how that prefix is excluded from the output
- how the function is called from the select

    style = libxslt.parseStylesheetDoc(styledoc)
    doc = libxml2.parseDoc("<doc/>")
    result = style.applyStylesheet(doc, { "bar": "'success'" })
    style.freeStylesheet()
    doc.freeDoc()

that part is identical, to the basic example except that the transformation is passed a dictionary of parameters. Note that the string passed "success" had to be quoted, otherwise it is interpreted as an XPath query for the childs of root named "success".

    root = result.children
    if root.name != "article":
        print "Unexpected root node name"
        sys.exit(1)
    if root.content != "SUCCESS":
        print "Unexpected root node content, extension function failed"
        sys.exit(1)
    if nodeName != 'article':
        print "The function callback failed to access its context"
        sys.exit(1)

    result.freeDoc()

That part just verifies that the transformation worked, that the parameter got properly passed to the engine, that the function f() got called and that it properly accessed the context to find the name of the insertion node.

### pyxsltproc.py:

this module is a bit too long to be described there but it is basically a rewrite of the xsltproc command line interface of libxslt in Python. It provides nearly all the functionalities of xsltproc and can be used as a base module to write Python customized XSLT processors. One of the thing to notice are:

    libxml2.lineNumbersDefault(1)
    libxml2.substituteEntitiesDefault(1)

those two calls in the main() function are needed to force the libxml2 processor to generate DOM trees compliant with the XPath data model.

Daniel Veillard