Extension functions for XPath and XSLT
======================================

This document describes how to use Python extension functions in XPath and
XSLT.  They allow you to do things like this::

  <xsl:value-of select="f:myPythonFunction(.//sometag)" />

Here is how such a function looks like.  As the first argument, it always
receives a dummy object.  It is currently None, but do not rely on this as it
may become meaningful in later versions of lxml.  The other arguments are
provided by the respective call in the XPath expression.  Any number of
arguments is allowed::

  >>> def hello(dummy, a):
  ...    return "Hello %s" % a
  >>> def ola(dummy, a):
  ...    return "Ola %s" % a


The FunctionNamespace
---------------------

In order to use a function in XPath/XSLT, it needs to have a (namespaced) name
by which it can be called during evaluation.  This is done using the
FunctionNamespace class.  For simplicity, we choose the empty namespace
(None)::

  >>> from lxml import etree
  >>> ns = etree.FunctionNamespace(None)
  >>> ns['hello'] = hello

This registers the function 'foo' with the name 'myfunction' in the default
namespace.  Now we're going to create a document that we can run XPath
expressions against::

  >>> from lxml import etree
  >>> from StringIO import StringIO
  >>> f = StringIO('<a><b>Haegar</b></a>')
  >>> doc = etree.parse(f)
  >>> root = doc.getroot()

Done. Now we can have XPath expressions call our new function::

  >>> print root.xpath("hello('world')")
  Hello world
  >>> print root.xpath('hello(local-name(*))')
  Hello b
  >>> print root.xpath('hello(string(b))')
  Hello Haegar

Note how we call both a Python function (hello) and an XPath built-in function
(local-name) in exactly the same way.  Normally, however, you would want to
separate the two in different namespaces.  The FunctionNamespace class allows
you to do this::

  >>> ns = etree.FunctionNamespace('http://mydomain.org/myfunctions')
  >>> ns['hello'] = hello
  >>> print root.xpath('f:hello(local-name(*))', {'f' : 'http://mydomain.org/myfunctions'})
  Hello b


Global prefix assignment
------------------------

In the last example, you had to specify a prefix for the function namespace.
If you always use the same prefix for a function namespace, you can also
register it with the namespace::

  >>> ns = etree.FunctionNamespace('http://mydomain.org/myother/functions')
  >>> ns.prefix = 'es'
  >>> ns['hello'] = ola
  >>> print root.xpath('es:hello(local-name(*))')
  Ola b

This is a global assignment, so take care not to assign the same prefix to
more than one namespace.  The resulting behaviour in that case is completely
undefined.  It is always a good idea to consistently use the same meaningful
prefix for each namespace throughout your application.

The prefix assignment only works with functions and FunctionNamespace objects,
not with the general Namespace object that registers element classes.  The
reasoning is that elements in lxml do not care about prefixes anyway, so it
would rather complicate things than be of any help.


What to return from a function
------------------------------

Extension functions can return any data type for which there is an XPath
equivalent.  This includes numbers, boolean values, elements and lists of
elements::

  >>> def returnsFloat(_):
  ...    return 1.7
  >>> def returnsBool(_):
  ...    return True
  >>> def returnFirstNode(_, nodes):
  ...    return nodes[0]

  >>> ns = etree.FunctionNamespace(None)
  >>> ns['float'] = returnsFloat
  >>> ns['bool']  = returnsBool
  >>> ns['first'] = returnFirstNode

  >>> e = etree.XPathEvaluator(doc)
  >>> e.evaluate("float()")
  1.7
  >>> e.evaluate("bool()")
  True
  >>> e.evaluate("count(first(//b))")
  1.0


Evaluators and XSLT
-------------------

Extension functions work for all ways of evaluating XPath expressions and for
XSLT execution::

  >>> e = etree.XPathEvaluator(doc)
  >>> print e.evaluate('es:hello(local-name(/a))')
  Ola a

  >>> e = etree.XPathEvaluator(doc, namespaces={'f' : 'http://mydomain.org/myfunctions'})
  >>> print e.evaluate('f:hello(local-name(/a))')
  Hello a

  >>> xslt = etree.XSLT(etree.ElementTree(etree.XML('''
  ...   <stylesheet version="1.0"
  ...          xmlns="http://www.w3.org/1999/XSL/Transform"
  ...          xmlns:es="http://mydomain.org/myother/functions">
  ...     <output method="text" encoding="ASCII"/>
  ...     <template match="/">
  ...       <value-of select="es:hello(string(//b))"/>
  ...     </template>
  ...   </stylesheet>
  ... ''')))
  >>> print xslt(doc)
  Ola Haegar

It is also possible to register namespaces with a single evaluator.  While the
following example involves no functions, the idea should still be clear::
  
  >>> f = StringIO('<hey:a xmlns:hey="http://mydomain.org/myfunctions" />')
  >>> ns_doc = etree.parse(f)
  >>> e = etree.XPathEvaluator(ns_doc)
  >>> e.evaluate('/a')
  []

This obviously returns nothing, but when we register the namespace with the
evaluator, we can access it via a prefix.  Note that this prefix mapping is
only known to this evaluator, as opposed to the global mapping of the
FunctionNamespace objects::

  >>> e.registerNamespace('foo', 'http://mydomain.org/myfunctions')
  >>> e.evaluate('/foo:a')[0].tag
  '{http://mydomain.org/myfunctions}a'


BETA Features
-------------

Note: the following features are still in beta state.  They may not work as
expected.

It is possible to return lists of newly created nodes as XML structures::

  >>> def returnsNodeSet(evaluator):
  ...    results = etree.Element('results')
  ...    result = etree.SubElement(results, 'result')
  ...    result.text = "Alpha"
  ...    result2 = etree.SubElement(results, 'result')
  ...    result2.text = "Beta"
  ...    result3 = etree.SubElement(results, 'result')
  ...    result3.text = "Gamma"
  ...    return [results]
  >>> extension4 = { (None, 'returnsNodeSet') : returnsNodeSet }
  >>> e = etree.XPathEvaluator(doc, None, extensions=[extension4])
  >>> r = e.evaluate("returnsNodeSet()")
  >>> len(r)
  1
  >>> t = r[0]
  >>> t.tag
  'results'
  >>> len(t)
  3
  >>> t[0].tag
  'result'
  >>> t[0].text
  'Alpha'
  >>> t[1].text
  'Beta'

It's even possible to filter that result set with another XPath expression::

  >>> r = e.evaluate("returnsNodeSet()/result")
  >>> len(r)
  3
  >>> r[0].tag
  'result'
  >>> r[1].tag
  'result'
  >>> r[0].text
  'Alpha'