Python extensions for XPath and XSLT

This document describes how to use Python extension functions in XPath and XSLT like this:

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

and extension elements in XSLT as in the following example:

<xsl:template match="*">
        <some-content />


XPath Extension functions

Here is how an extension function looks like. As the first argument, it always receives a context object (see below). The other arguments are provided by the respective call in the XPath expression, one in the following examples. Any number of arguments is allowed:

>>> def hello(context, a):
...    return "Hello %s" % a
>>> def ola(context, a):
...    return "Ola %s" % a
>>> def loadsofargs(context, *args):
...    return "Got %d arguments." % len(args)

The FunctionNamespace

In order to use a function in XPath or 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
>>> ns['countargs'] = loadsofargs

This registers the function hello with the name hello in the default namespace (None), and the function loadsofargs with the name countargs. Now we're going to create a document that we can run XPath expressions against:

>>> root = etree.XML('<a><b>Haegar</b></a>')
>>> doc = etree.ElementTree(root)

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

>>> print(root.xpath("hello('Dr. Falken')"))
Hello Dr. Falken
>>> print(root.xpath('hello(local-name(*))'))
Hello b
>>> print(root.xpath('hello(string(b))'))
Hello Haegar
>>> print(root.xpath('countargs(., b, ./*)'))
Got 3 arguments.

Note how we call both a Python function (hello) and an XPath built-in function (string) 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('')
>>> ns['hello'] = hello
>>> prefixmap = {'f' : ''}
>>> print(root.xpath('f:hello(local-name(*))', namespaces=prefixmap))
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('')
>>> 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.

The XPath context

Functions get a context object as first parameter. In lxml 1.x, this value was None, but since lxml 2.0 it provides two properties: eval_context and context_node. The context node is the Element where the current function is called:

>>> def print_tag(context, nodes):
...     print("%s: %s" % (context.context_node.tag, [ n.tag for n in nodes ]))

>>> ns = etree.FunctionNamespace('')
>>> ns.prefix = "pt"
>>> ns["print_tag"] = print_tag

>>> ignore = root.xpath("//*[pt:print_tag(.//*)]")
a: ['b']
b: []

The eval_context is a dictionary that is local to the evaluation. It allows functions to keep state:

>>> def print_context(context):
...     context.eval_context[context.context_node.tag] = "done"
...     print(sorted(context.eval_context.items()))
>>> ns["print_context"] = print_context

>>> ignore = root.xpath("//*[pt:print_context()]")
[('a', 'done')]
[('a', 'done'), ('b', 'done')]

Evaluators and XSLT

Extension functions work for all ways of evaluating XPath expressions and for XSL transformations:

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

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

>>> xslt = etree.XSLT(etree.XML('''
...   <stylesheet version="1.0"
...          xmlns=""
...          xmlns:es="">
...     <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 after its creation. While the following example involves no functions, the idea should still be clear:

>>> f = StringIO('<a xmlns="" />')
>>> ns_doc = etree.parse(f)
>>> e = etree.XPathEvaluator(ns_doc)
>>> e('/a')

This returns nothing, as we did not ask for the right namespace. When we register the namespace with the evaluator, however, we can access it via a prefix:

>>> e.register_namespace('foo', '')
>>> e('/foo:a')[0].tag

Note that this prefix mapping is only known to this evaluator, as opposed to the global mapping of the FunctionNamespace objects:

>>> e2 = etree.XPathEvaluator(ns_doc)
>>> e2('/foo:a')
Traceback (most recent call last):
lxml.etree.XPathEvalError: Undefined namespace prefix

Evaluator-local extensions

Apart from the global registration of extension functions, there is also a way of making extensions known to a single Evaluator or XSLT. All evaluators and the XSLT object accept a keyword argument extensions in their constructor. The value is a dictionary mapping (namespace, name) tuples to functions:

>>> extensions = {('local-ns', 'local-hello') : hello}
>>> namespaces = {'l' : 'local-ns'}

>>> e = etree.XPathEvaluator(doc, namespaces=namespaces, extensions=extensions)
>>> print(e('l:local-hello(string(b))'))
Hello Haegar

For larger numbers of extension functions, you can define classes or modules and use the Extension helper:

>>> class MyExt:
...     def function1(self, _, arg):
...         return '1'+arg
...     def function2(self, _, arg):
...         return '2'+arg
...     def function3(self, _, arg):
...         return '3'+arg

>>> ext_module = MyExt()
>>> functions = ('function1', 'function2')
>>> extensions = etree.Extension( ext_module, functions, ns='local-ns' )

>>> e = etree.XPathEvaluator(doc, namespaces=namespaces, extensions=extensions)
>>> print(e('l:function1(string(b))'))

The optional second argument to Extension can either be be a sequence of names to select from the module, a dictionary that explicitly maps function names to their XPath alter-ego or None (explicitly passed) to take all available functions under their original name (if their name does not start with '_').

The additional ns keyword argument takes a namespace URI or None (also if left out) for the default namespace. The following examples will therefore all do the same thing:

>>> functions = ('function1', 'function2', 'function3')
>>> extensions = etree.Extension( ext_module, functions )
>>> e = etree.XPathEvaluator(doc, extensions=extensions)
>>> print(e('function1(function2(function3(string(b))))'))

>>> extensions = etree.Extension( ext_module, functions, ns=None )
>>> e = etree.XPathEvaluator(doc, extensions=extensions)
>>> print(e('function1(function2(function3(string(b))))'))

>>> extensions = etree.Extension(ext_module)
>>> e = etree.XPathEvaluator(doc, extensions=extensions)
>>> print(e('function1(function2(function3(string(b))))'))

>>> functions = {
...     'function1' : 'function1',
...     'function2' : 'function2',
...     'function3' : 'function3'
...     }
>>> extensions = etree.Extension(ext_module, functions)
>>> e = etree.XPathEvaluator(doc, extensions=extensions)
>>> print(e('function1(function2(function3(string(b))))'))

For convenience, you can also pass a sequence of extensions:

>>> extensions1 = etree.Extension(ext_module)
>>> extensions2 = etree.Extension(ext_module, ns='local-ns')
>>> e = etree.XPathEvaluator(doc, extensions=[extensions1, extensions2],
...                          namespaces=namespaces)
>>> print(e('function1(l:function2(function3(string(b))))'))

What to return from a function

Extension functions can return any data type for which there is an XPath equivalent (see the documentation on XPath return values). This includes numbers, boolean values, elements and lists of elements. Note that integers will also be returned as floats:

>>> def returnsFloat(_):
...    return 1.7
>>> def returnsInteger(_):
...    return 1
>>> def returnsBool(_):
...    return True
>>> def returnFirstNode(_, nodes):
...    return nodes[0]

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

>>> e = etree.XPathEvaluator(doc)
>>> e("float()")
>>> e("int()")
>>> int( e("int()") )
>>> e("bool()")
>>> e("count(first(//b))")

As the last example shows, you can pass the results of functions back into the XPath expression. Elements and sequences of elements are treated as XPath node-sets:

>>> def returnsNodeSet(_):
...     results1 = etree.Element('results1')
...     etree.SubElement(results1, 'result').text = "Alpha"
...     etree.SubElement(results1, 'result').text = "Beta"
...     results2 = etree.Element('results2')
...     etree.SubElement(results2, 'result').text = "Gamma"
...     etree.SubElement(results2, 'result').text = "Delta"
...     results3 = etree.SubElement(results2, 'subresult')
...     return [results1, results2, results3]

>>> ns['new-node-set'] = returnsNodeSet

>>> e = etree.XPathEvaluator(doc)

>>> r = e("new-node-set()/result")
>>> print([ t.text for t in r ])
['Alpha', 'Beta', 'Gamma', 'Delta']

>>> r = e("new-node-set()")
>>> print([ t.tag for t in r ])
['results1', 'results2', 'subresult']
>>> print([ len(t) for t in r ])
[2, 3, 0]
>>> r[0][0].text

>>> etree.tostring(r[0])

>>> etree.tostring(r[1])

>>> etree.tostring(r[2])

The current implementation deep-copies newly created elements in node-sets. Only the elements and their children are passed on, no outlying parents or tail texts will be available in the result. This also means that in the above example, the subresult elements in results2 and results3 are no longer identical within the node-set, they belong to independent trees:

>>> print("%s - %s" % (r[1][-1].tag, r[2].tag))
subresult - subresult
>>> print(r[1][-1] == r[2])
>>> print(r[1][-1].getparent().tag)
>>> print(r[2].getparent())

This is an implementation detail that you should be aware of, but you should avoid relying on it in your code. Note that elements taken from the source document (the most common case) do not suffer from this restriction. They will always be passed unchanged.

XSLT extension elements

Just like the XPath extension functions described above, lxml supports custom extension elements in XSLT. This means, you can write XSLT code like this:

<xsl:template match="*">
        <some-content />

And then you can implement the element in Python like this:

>>> class MyExtElement(etree.XSLTExtension):
...     def execute(self, context, self_node, input_node, output_parent):
...         print("Hello from XSLT!")
...         output_parent.text = "I did it!"
...         # just copy own content input to output
...         output_parent.extend( list(self_node) )

The arguments passed to the .execute() method are

The opaque evaluation context. You need this when calling back into the XSLT processor.
A read-only Element object that represents the extension element in the stylesheet.
The current context Element in the input document (also read-only).
The current insertion point in the output document. You can append elements or set the text value (not the tail). Apart from that, the Element is read-only.

Declaring extension elements

In XSLT, extension elements can be used like any other XSLT element, except that they must be declared as extensions using the standard XSLT extension-element-prefixes option:

>>> xslt_ext_tree = etree.XML('''
... <xsl:stylesheet version="1.0"
...     xmlns:xsl=""
...     xmlns:my="testns"
...     extension-element-prefixes="my">
...     <xsl:template match="/">
...         <foo><my:ext><child>XYZ</child></my:ext></foo>
...     </xsl:template>
...     <xsl:template match="child">
...         <CHILD>--xyz--</CHILD>
...     </xsl:template>
... </xsl:stylesheet>''')

To register the extension, add its namespace and name to the extension mapping of the XSLT object:

>>> my_extension = MyExtElement()
>>> extensions = { ('testns', 'ext') : my_extension }
>>> transform = etree.XSLT(xslt_ext_tree, extensions = extensions)

Note how we pass an instance here, not the class of the extension. Now we can run the transformation and see how our extension is called:

>>> root = etree.XML('<dummy/>')
>>> result = transform(root)
Hello from XSLT!
>>> str(result)
'<?xml version="1.0"?>\n<foo>I did it!<child>XYZ</child></foo>\n'

Applying XSL templates

XSLT extensions are a very powerful feature that allows you to interact directly with the XSLT processor. You have full read-only access to the input document and the stylesheet, and you can even call back into the XSLT processor to process templates. Here is an example that passes an Element into the .apply_templates() method of the XSLTExtension instance:

>>> class MyExtElement(etree.XSLTExtension):
...     def execute(self, context, self_node, input_node, output_parent):
...         child = self_node[0]
...         results = self.apply_templates(context, child)
...         output_parent.append(results[0])

>>> my_extension = MyExtElement()
>>> extensions = { ('testns', 'ext') : my_extension }
>>> transform = etree.XSLT(xslt_ext_tree, extensions = extensions)

>>> root = etree.XML('<dummy/>')
>>> result = transform(root)
>>> str(result)
'<?xml version="1.0"?>\n<foo><CHILD>--xyz--</CHILD></foo>\n'

Here, we applied the templates to a child of the extension element itself, i.e. to an element inside the stylesheet instead of an element of the input document.

The return value of .apply_templates() is always a list. It may contain a mix of elements and strings, collected from the XSLT processing result. If you want to append these values to the output parent, be aware that you cannot use the .append() method to add strings. In many cases, you would only be interested in elements anyway, so you can discard strings (e.g. formatting whitespace) and append the rest.

If you want to include string results in the output, you can either build an appropriate tree yourself and append that, or you can manually add the string values to the current output tree, e.g. by concatenating them with the .tail of the last element that was appended.

Note that you can also let lxml build the result tree for you by passing the output_parent into the .apply_templates() method. In this case, the result will be None and all content found by applying templates will be appended to the output parent.

If you do not care about string results at all, e.g. because you already know that they will only contain whitespace, you can pass the option elements_only=True to the .apply_templates() method, or pass remove_blank_text=True to remove only those strings that consist entirely of whitespace.

Working with read-only elements

There is one important thing to keep in mind: all Elements that the execute() method gets to deal with are read-only Elements, so you cannot modify them. They also will not easily work in the API. For example, you cannot pass them to the tostring() function or wrap them in an ElementTree.

What you can do, however, is to deepcopy them to make them normal Elements, and then modify them using the normal etree API. So this will work:

>>> from copy import deepcopy
>>> class MyExtElement(etree.XSLTExtension):
...     def execute(self, context, self_node, input_node, output_parent):
...         child = deepcopy(self_node[0])
...         child.text = "NEW TEXT"
...         output_parent.append(child)

>>> my_extension = MyExtElement()
>>> extensions = { ('testns', 'ext') : my_extension }
>>> transform = etree.XSLT(xslt_ext_tree, extensions = extensions)

>>> root = etree.XML('<dummy/>')
>>> result = transform(root)
>>> str(result)
'<?xml version="1.0"?>\n<foo><child>NEW TEXT</child></foo>\n'