Using custom Element classes in lxml

lxml has very sophisticated support for custom Element classes. You can provide your own classes for Elements and have lxml use them by default, for all elements generated by a specific parser, for a specific tag name in a specific namespace or for an exact element at a specific position in the tree.

Custom Elements must inherit from the lxml.etree.ElementBase class, which provides the Element interface for subclasses:

>>> from lxml import etree
>>> class HonkElement(etree.ElementBase):
...    def honking(self):
...       return self.get('honking') == 'true'
...    honking = property(honking)

This defines a new Element class HonkElement with a property honking.

Note that you cannot (or rather must not) instantiate this class yourself. lxml.etree will do that for you through its normal ElementTree API.

Contents

Element initialization
Setting up a class lookup scheme
Implementing namespaces

Element initialization

There is one thing to know up front. Element classes must not have a constructor, neither must there be any internal state (except for the data stored in the underlying XML tree). Element instances are created and garbage collected at need, so there is no way to predict when and how often a constructor would be called. Even worse, when the __init__ method is called, the object may not even be initialized yet to represent the XML tag, so there is not much use in providing an __init__ method in subclasses.

Most use cases will not require any class initialisation, so you can content yourself with skipping to the next section for now. However, if you really need to set up your element class on instantiation, there is one possible way to do so. ElementBase classes have an _init() method that can be overridden. It can be used to modify the XML tree, e.g. to construct special children or verify and update attributes.

The semantics of _init() are as follows:

It is called at least once on element instantiation time. That is, when a Python representation of the element is created by lxml. At that time, the element object is completely initialized to represent a specific XML element within the tree.
The method has complete access to the XML tree. Modifications can be done in exactly the same way as anywhere else in the program.
Python representations of elements may be created multiple times during the lifetime of an XML element in the underlying tree. The _init() code provided by subclasses must take special care by itself that multiple executions either are harmless or that they are prevented by some kind of flag in the XML tree. The latter can be achieved by modifying an attribute value or by removing or adding a specific child node and then verifying this before running through the init process.
Any exceptions raised in _init() will be propagated throught the API call that lead to the creation of the Element. So be careful with the code you write here as its exceptions may turn up in various unexpected places.

Setting up a class lookup scheme

The first thing to do when deploying custom element classes is to register a class lookup scheme on a parser. lxml.etree provides quite a number of different schemes, that also support class lookup based on namespaces or attribute values. Most lookups support fallback chaining, which allows the next lookup mechanism to take over when the previous one fails to find a class.

For example, setting a different default element class for a parser works as follows:

>>> parser_lookup = etree.ElementDefaultClassLookup(element=HonkElement)
>>> parser = etree.XMLParser()
>>> parser.setElementClassLookup(parser_lookup)

There is one drawback of the parser based scheme: the Element() factory does not know about your specialised parser and creates a new document that deploys the default parser:

>>> el = etree.Element("root")
>>> print isinstance(el, HonkElement)
False

You should therefore avoid using this function in code that uses custom classes. The makeelement() method of parsers provides a simple replacement:

>>> el = parser.makeelement("root")
>>> print isinstance(el, HonkElement)
True

If you use a parser at the module level, you can easily redirect a module level Element() factory to the parser method by adding code like this:

>>> MODULE_PARSER = etree.XMLParser()
>>> Element = MODULE_PARSER.makeelement

While the XML() and HTML() factories also depend on the default parser, you can pass them a different parser as second argument:

>>> element = etree.XML("<test/>")
>>> print isinstance(element, HonkElement)
False

>>> element = etree.XML("<test/>", parser)
>>> print isinstance(element, HonkElement)
True

Whenever you create a document with a parser, it will inherit the lookup scheme and all subsequent element instantiations for this document will use it:

>>> element = etree.fromstring("<test/>", parser)
>>> print isinstance(element, HonkElement)
True
>>> el = etree.SubElement(element, "subel")
>>> print isinstance(el, HonkElement)
True

For small projects, you may also consider setting a lookup scheme on the default parser. To avoid interfering with other modules, however, it is usually a better idea to use a dedicated parser for each module (or a parser pool when using threads) and then register the required lookup scheme only for this parser.

Default class lookup

This is the most simple lookup mechanism. It always returns the default element class. Consequently, no further fallbacks are supported, but this scheme is a good fallback for other custom lookup mechanisms.

Usage:

>>> lookup = etree.ElementDefaultClassLookup()
>>> parser = etree.XMLParser()
>>> parser.setElementClassLookup(lookup)

Note that the default for new parsers is to use the global fallback, which is also the default lookup (if not configured otherwise).

To change the default element implementation, you can pass your new class to the constructor. While it accepts classes for element, comment and pi nodes, most use cases will only override the element class:

>>> el = parser.makeelement("myelement")
>>> print isinstance(el, HonkElement)
False

>>> lookup = etree.ElementDefaultClassLookup(element=HonkElement)
>>> parser.setElementClassLookup(lookup)

>>> el = parser.makeelement("myelement")
>>> print isinstance(el, HonkElement)
True
>>> el.honking
False
>>> el = parser.makeelement("myelement", honking='true')
>>> print etree.tostring(el)
<myelement honking="true"/>
>>> el.honking
True

Namespace class lookup

This is an advanced lookup mechanism that supports namespace/tag-name specific element classes. You can select it by calling:

>>> lookup = etree.ElementNamespaceClassLookup()
>>> parser = etree.XMLParser()
>>> parser.setElementClassLookup(lookup)

See the separate section on implementing namespaces below to learn how to make use of it.

This scheme supports a fallback mechanism that is used in the case where the namespace is not found or no class was registered for the element name. Normally, the default class lookup is used here. To change it, pass the desired fallback lookup scheme to the constructor:

>>> fallback = etree.ElementDefaultClassLookup(element=HonkElement)
>>> lookup = etree.ElementNamespaceClassLookup(fallback)
>>> parser.setElementClassLookup(lookup)

Attribute based lookup

This scheme uses a mapping from attribute values to classes. An attribute name is set at initialisation time and is then used to find the corresponding value. It is set up as follows:

>>> id_class_mapping = {} # maps attribute values to element classes
>>> lookup = etree.AttributeBasedElementClassLookup(
...                                      'id', id_class_mapping)
>>> parser = etree.XMLParser()
>>> parser.setElementClassLookup(lookup)

Instead of a global setup of this scheme, you should consider using a per-parser setup.

This class uses its fallback if the attribute is not found or its value is not in the mapping. Normally, the default class lookup is used here. If you want to use the namespace lookup, for example, you can use this code:

>>> fallback = etree.ElementNamespaceClassLookup()
>>> lookup = etree.AttributeBasedElementClassLookup(
...                       'id', id_class_mapping, fallback)
>>> parser = etree.XMLParser()
>>> parser.setElementClassLookup(lookup)

Custom element class lookup

This is the most customisable way of finding element classes on a per-element basis. It allows you to implement a custom lookup scheme in a subclass:

>>> class MyLookup(etree.CustomElementClassLookup):
...     def lookup(self, node_type, document, namespace, name):
...         return MyElementClass # defined elsewhere

>>> parser = etree.XMLParser()
>>> parser.setElementClassLookup(MyLookup())

The lookup() method must either return None (which triggers the fallback mechanism) or a subclass of lxml.etree.ElementBase. It can take any decision it wants based on the node type (one of "element", "comment", "PI"), the XML document of the element, or its namespace or tag name.

Instead of a global setup of this scheme, you should consider using a per-parser setup.

Tree based element class lookup in Python

Taking more elaborate decisions than allowed by the custom scheme is difficult to achieve in pure Python. It would require access to the tree - before the elements in the tree have been instantiated as Python Element objects.

Luckily, there is a way to do this. The separate module lxml.pyclasslookup provides a lookup class called PythonElementClassLookup that works similar to the custom lookup scheme:

>>> from lxml.pyclasslookup import PythonElementClassLookup
>>> class MyLookup(PythonElementClassLookup):
...     def lookup(self, document, element):
...         return MyElementClass # defined elsewhere

>>> parser = etree.XMLParser()
>>> parser.setElementClassLookup(MyLookup())

As before, the first argument to the lookup() method is the opaque document instance that contains the Element. The second arguments is a lightweight Element proxy implementation that is only valid during the lookup. Do not try to keep a reference to it. Once the lookup is finished, the proxy will become invalid. You will get an AssertionError if you access any of the properties or methods outside the scope of the lookup call where they were instantiated.

During the lookup, the element object behaves mostly like a normal Element instance. It provides the properties tag, text, tail etc. and supports indexing, slicing and the getchildren(), getparent() etc. methods. It does not support iteration, nor does it support any kind of modification. All of its properties are read-only and it cannot be removed or inserted into other trees. You can use it as a starting point to freely traverse the tree and collect any kind of information that its elements provide. Once you have taken the decision which class to use for this element, you can simply return it and have lxml take care of cleaning up the instantiated proxy classes.

Implementing namespaces

lxml allows you to implement namespaces, in a rather literal sense. After setting up the namespace class lookup mechanism as described above, you can build a new element namespace (or retrieve an existing one) by calling the Namespace class:

>>> lookup = etree.ElementNamespaceClassLookup()
>>> parser = etree.XMLParser()
>>> parser.setElementClassLookup(lookup)

>>> namespace = etree.Namespace('http://hui.de/honk')

and then register the new element type with that namespace, say, under the tag name honk:

>>> namespace['honk'] = HonkElement

After this, you create and use your XML elements through the normal API of lxml:

>>> xml = '<honk xmlns="http://hui.de/honk" honking="true"/>'
>>> honk_element = etree.XML(xml, parser)
>>> print honk_element.honking
True

The same works when creating elements by hand:

>>> honk_element = parser.makeelement('{http://hui.de/honk}honk',
...                                   honking='true')
>>> print honk_element.honking
True

Essentially, what this allows you to do, is to give elements a custom API based on their namespace and tag name.

A somewhat related topic are extension functions which use a similar mechanism for registering extension functions in XPath and XSLT.

In the Namespace example above, we associated the HonkElement class only with the 'honk' element. If an XML tree contains different elements in the same namespace, they do not pick up the same implementation:

>>> xml = '<honk xmlns="http://hui.de/honk" honking="true"><bla/></honk>'
>>> honk_element = etree.XML(xml, parser)
>>> print honk_element.honking
True
>>> print honk_element[0].honking
Traceback (most recent call last):
...
AttributeError: 'etree._Element' object has no attribute 'honking'

You can therefore provide one implementation per element name in each namespace and have lxml select the right one on the fly. If you want one element implementation per namespace (ignoring the element name) or prefer having a common class for most elements except a few, you can specify a default implementation for an entire namespace by registering that class with the empty element name (None).

You may consider following an object oriented approach here. If you build a class hierarchy of element classes, you can also implement a base class for a namespace that is used if no specific element class is provided. Again, you can just pass None as an element name:

>>> class HonkNSElement(etree.ElementBase):
...    def honk(self):
...       return "HONK"
>>> namespace[None] = HonkNSElement

>>> class HonkElement(HonkNSElement):
...    def honking(self):
...       return self.get('honking') == 'true'
...    honking = property(honking)
>>> namespace['honk'] = HonkElement

Now you can rely on lxml to always return objects of type HonkNSElement or its subclasses for elements of this namespace:

>>> xml = '<honk xmlns="http://hui.de/honk" honking="true"><bla/></honk>'
>>> honk_element = etree.XML(xml, parser)

>>> print type(honk_element), type(honk_element[0])
<class 'HonkElement'> <class 'HonkNSElement'>

>>> print honk_element.honking
True
>>> print honk_element.honk()
HONK
>>> print honk_element[0].honk()
HONK
>>> print honk_element[0].honking
Traceback (most recent call last):
...
AttributeError: 'HonkNSElement' object has no attribute 'honking'