- lxml

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# 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, only for a specific tag name in a specific namespace or even 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 honk(etree.ElementBase):
    ...    def honking(self):
    ...       return self.get('honking') == 'true'
    ...    honking = property(honking)

This defines a new Element class `honk` with a property `honking`.

The following document describes how you can make lxml.etree use these custom Element classes.

Contents

- Background on Element proxies
- Element initialization
- Setting up a class lookup scheme
  - Default class lookup
  - Namespace class lookup
  - Attribute based lookup
  - Custom element class lookup
  - Tree based element class lookup in Python
- Generating XML with custom classes
- Implementing namespaces

# Background on Element proxies

Being based on libxml2, lxml.etree holds the entire XML tree in a C structure. To communicate with Python code, it creates Python proxy objects for the XML elements on demand.

The mapping between C elements and Python Element classes is completely configurable. When you ask lxml.etree for an Element by using its API, it will instantiate your classes for you. All you have to do is tell lxml which class to use for which kind of Element. This is done through a class lookup scheme, as described in the sections below.

# Element initialization

There is one thing to know up front. Element classes must not have an `__init___` or `__new__` method. There should not be any internal state either, 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 proxy is created for them. Even worse, when the `__init__` method is called, the object is not even 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 once on Element class 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 C 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 the `honk` Element as a default element class for a parser works as follows:

    >>> parser_lookup = etree.ElementDefaultClassLookup(element=honk)
    >>> parser = etree.XMLParser()
    >>> parser.set_element_class_lookup(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, honk))
    False

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

    >>> el = parser.makeelement("root")
    >>> print(isinstance(el, honk))
    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_level_parser = etree.XMLParser()
    >>> Element = module_level_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, honk))
    False

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

For testing code in the Python interpreter and 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 nice fallback for other custom lookup mechanisms.

Usage:

    >>> lookup = etree.ElementDefaultClassLookup()
    >>> parser = etree.XMLParser()
    >>> parser.set_element_class_lookup(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, honk))
    False

    >>> lookup = etree.ElementDefaultClassLookup(element=honk)
    >>> parser.set_element_class_lookup(lookup)

    >>> el = parser.makeelement("myelement")
    >>> print(isinstance(el, honk))
    True
    >>> el.honking
    False
    >>> el = parser.makeelement("myelement", honking='true')
    >>> etree.tostring(el)
    b'<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.set_element_class_lookup(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=honk)
    >>> lookup = etree.ElementNamespaceClassLookup(fallback)
    >>> parser.set_element_class_lookup(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 in a dictionary. It is set up as follows:

    >>> id_class_mapping = {'1234' : honk} # maps attribute values to classes

    >>> lookup = etree.AttributeBasedElementClassLookup(
    ...                                      'id', id_class_mapping)
    >>> parser = etree.XMLParser()
    >>> parser.set_element_class_lookup(lookup)

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.set_element_class_lookup(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 honk # be a bit more selective here ...

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

The `.lookup()` method must return either 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", "entity"), the XML document of the element, or its namespace or tag name.

## Tree based element class lookup in Python

Taking more elaborate decisions than allowed by the custom scheme is difficult to achieve in pure Python, as it results in a chicken-and-egg problem. It would require access to the tree - before the elements in the tree have been instantiated as Python Element proxies.

Luckily, there is a way to do this. The `PythonElementClassLookup` works similar to the custom lookup scheme:

    >>> class MyLookup(etree.PythonElementClassLookup):
    ...     def lookup(self, document, element):
    ...         return MyElementClass # defined elsewhere

    >>> parser = etree.XMLParser()
    >>> parser.set_element_class_lookup(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.

Sidenote: this lookup scheme originally lived in a separate module called `lxml.pyclasslookup`.

# Generating XML with custom classes

Up to lxml 2.1, you could not instantiate proxy classes yourself. Only lxml.etree could do that when creating an object representation of an existing XML element. Since lxml 2.2, however, instantiating this class will simply create a new Element:

    >>> el = honk(honking = 'true')
    >>> el.tag
    'honk'
    >>> el.honking
    True

Note, however, that the proxy you create here will be garbage collected just like any other proxy. You can therefore not count on lxml.etree using the same class that you instantiated when you access this Element a second time after letting its reference go. You should therefore always use a corresponding class lookup scheme that returns your Element proxy classes for the elements that they create. The `ElementNamespaceClassLookup` is generally a good match.

You can use custom Element classes to quickly create XML fragments:

    >>> class hale(etree.ElementBase): pass
    >>> class bopp(etree.ElementBase): pass

    >>> el = hale( "some ", honk(honking = 'true'), bopp, " text" )

    >>> print(etree.tostring(el, encoding=unicode))
    <hale>some <honk honking="true"/><bopp/> text</hale>

# 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 `get_namespace(uri)` method of the lookup:

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

    >>> namespace = lookup.get_namespace('http://hui.de/honk')

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

    >>> namespace['honk'] = honk

If you have many Element classes declared in one module, and they are all named like the elements they create, you can simply use `namespace.update(vars())` at the end of your module to declare them automatically. The implementation is smart enough to ignore everything that is not an Element class.

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 setup example above, we associated the `honk` Element 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)
    ...
    AttributeError: 'lxml.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 # default Element for namespace

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

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

    >>> print(honk_element.honking)
    True
    >>> print(honk_element.honk())
    HONK

    >>> print(honk_element[0].honk())
    HONK
    >>> print(honk_element[0].honking)
    ...
    AttributeError: 'HonkNSElement' object has no attribute 'honking'