Advanced

Namespaces

xml4h supports using XML namespaces in a number of ways, and tries to make this sometimes complex and fiddly aspect of XML a little easier to deal with.

Namespace URIs

XML document nodes can be associated with a namespace URI which uniquely identifies the namespace. At bottom a URI is really just a name to identifiy the namespace, which may or may not point at an actual resource.

Namespace URIs are the core piece of the namespacing puzzle, everything else is extras.

Namespace URI values are assigned to a node in one of three ways:

  • an xmlns attribute on an element assigns a namespace URI to that element, and may also define a shorthand prefix for the namespace:

    <AnElement xmlns:my-prefix="urn:example-uri">

    Note

    Technically the xmlns attribute must itself also be in the special XML namespacing namespace http://www.w3.org/2000/xmlns/. You needn’t care about this.

  • a tag or attribute name includes a prefix alias portion that specifies the namespace the item belongs to:

    <my-prefix:AnotherElement attr1="x" my-prefix:attr2="i am namespaced">

    A prefix alias can be defined using an “xmlns” attribute as described above, or by using the Builder ns_prefix() or Node set_ns_prefix() methods.

  • in an apparent effort to reduce confusion around namespace URIs and prefixes, some XML libraries avoid prefix aliases altogether and instead require you to specify the full namespace URI as a prefix to tag and attribute names using a special syntax with braces:

    >>> tagname = '{urn:example-uri}YetAnotherWayToNamespace'
    

    Note

    In the author’s opinion, using a non-standard way to define namespaces does not reduce confusion. xml4h supports this approach technically but not philosphically.

xml4h allows you to assign namespace URIs to document nodes when using the Builder:

>>> # Assign a default namespace with ns_uri
>>> import xml4h
>>> b = xml4h.build('Doc', ns_uri='ns-uri')
>>> root = b.root

>>> # Descendent without a namespace inherit their ancestor's default one
>>> elem1 = b.elem('Elem1').dom_element
>>> elem1.namespace_uri
'ns-uri'

>>> # Define a prefix alias to assign a new or existing namespace URI
>>> elem2 = b.ns_prefix('my-ns', 'second-ns-uri') \
...     .elem('my-ns:Elem2').dom_element
>>> print root.xml()
<Doc xmlns="ns-uri" xmlns:my-ns="second-ns-uri">
    <Elem1/>
    <my-ns:Elem2/>
</Doc>

>>> # Or use the explicit URI prefix approach, if you must
>>> elem3 = b.elem('{third-ns-uri}Elem3').dom_element
>>> elem3.namespace_uri
'third-ns-uri'

And when adding nodes with the API:

>>> # Define the ns_uri argument when creating a new element
>>> elem4 = root.add_element('Elem4', ns_uri='fourth-ns-uri')

>>> # Attributes can be namespaced too
>>> elem4.set_attributes({'my-ns:attr1': 'value'})

>>> print elem4.xml()
<Elem4 my-ns:attr1="value" xmlns="fourth-ns-uri"/>

Filtering by Namespace

xml4h allows you to find and filter nodes based on their namespace.

The find() method takes a ns_uri keyword argument to return only elements in that namespace:

>>> # By default, find ignores namespaces...
>>> [n.local_name for n in root.find()]
[u'Elem1', u'Elem2', u'Elem3', u'Elem4']
>>> # ...but will filter by namespace URI if you wish
>>> [n.local_name for n in root.find(ns_uri='fourth-ns-uri')]
[u'Elem4']

Similarly, a node’s children listing can be filtered:

>>> len(root.children)
4
>>> root.children(ns_uri='ns-uri')
[<xml4h.nodes.Element: "Elem1">]

XPath queries can also filter by namespace, but the xpath() method needs to be given a dictionary mapping of prefix aliases to URIs:

>>> root.xpath('//ns4:*', namespaces={'ns4': 'fourth-ns-uri'})
[<xml4h.nodes.Element: "Elem4">]

Note

Normally, because XPath queries rely on namespace prefix aliases, they cannot find namespaced nodes in the default namespace which has an “empty” prefix name. xml4h works around this limitation by providing the special empty/default prefix alias ‘_’.

Element Names: Local and Prefix Components

When you use a namespace prefix alias to define the namespace an element or attribute belongs to, the name of that node will be made up of two components:

  • prefix - the namespace alias.
  • local - the real name of the node, without the namespace alias.

xml4h makes the full (qualified) name, and the two components, available at node attributes:

>>> # Elem2's namespace was defined earlier using a prefix alias
>>> elem2
<xml4h.nodes.Element: "my-ns:Elem2">

# The full node name...
>>> elem2.name
u'my-ns:Elem2'
>>> # ...comprises a prefix...
>>> elem2.prefix
u'my-ns'
>>> # ...and a local name component
>>> elem2.local_name
u'Elem2'

>>> # Here is an element without a prefix alias
>>> elem1.name
u'Elem1'
>>> elem1.prefix == None
True
>>> elem1.local_name
u'Elem1'

xml4h Architecture

To best understand the xml4h library and to use it appropriately in demanding situations, you should appreciate what the library is not.

xml4h is not a full-fledged XML library in its own right, far from it. Instead of implementing low-level document parsing and manipulation tools, it operates as an abstraction layer on top of the pre-existing XML processing libraries you already know.

This means the improved API and tool suite provided by xml4h work by mediating operations you perform, asking the underlying XML library to do the work, and packaging up the results of this work as wrapped xml4h objects.

This approach has a number of implications, good and bad.

On the good side:

  • you can start using and benefiting from xml4h in an existing projects that already use a supported XML library without any impact, it can fit right in.
  • xml4h can take advantage of the existing powerful and fast XML libraries to do its work.
  • by providing an abstraction layer over multiple libraries, xml4h can make it (relatively) easy to switch the underlying library without you needing to rewrite your own XML handling code.
  • by building on the shoulders of giants, xml4h itself can remain relatively lightweight and focussed on simplicity and usability.
  • the author of xml4h does not have to write XML-handling code in C...

On the bad side:

  • if the underlying XML libraries available in the Python environment do not support a feature (like XPath querying) then that feature will not be available in xml4h.
  • xml4h cannot provide radical new XML processing features, since the bulk of its work must be done by the underlying library.
  • the abstraction layer xml4h uses to do its work requires more resources than it would to use the underlying library directly, so if you absolutely need maximal speed or minimal memory use the library might prove too expensive.
  • xml4h sometimes needs to jump through some hoops to maintain the shared abstraction interface over multiple libraries, which means extra work is done in Python instead of by the underlying library code in C.

The author believes the benefits of using xml4h outweighs the drawbacks in the majority of real-world situations, or he wouldn’t have created the library in the first place, but ultimately it is up to you to decide where you should or should not use it.

Library Adapters

To provide an abstraction layer over multiple underlying XML libraries, xml4h uses an “adapter” mechanism to mediate operations on documents. There is an adapter implementation for each library xml4h can work with, each of which extends the XmlImplAdapter class. This base class includes some standard behaviour, and defines the interface for adapter implementations (to the extent you can define such interfaces in Python).

The current version of xml4h includes adapter implementations for the three main XML processing libraries for Python:

  • LXMLAdapter works with the excellent lxml library which is very full-featured and fast, but which is not included in the standard library.
  • cElementTreeAdapter and ElementTreeAdapter work with the ElementTree libraries included with the standard library of Python versions 2.7 and later. ElementTree is fast and includes support for some basic XPath expressions. If the C-based version of ElementTree is available, the former adapter is made available and should be used for best performance.
  • XmlDomImplAdapter works with the minidom W3C-style XML library included with the standard library. This library is always available but is slower and has fewer features than alternative libraries (e.g. no support for XPath)

The adapter layer allows the rest of the xml4h library code to remain almost entirely oblivious to the underlying XML library that happens to be available at the time. The xml4h Builder, Node objects, writer etc. call adapter methods to perform document operations, and the adapter is responsible for doing the necessary work with the underlying library.

“Best” Adapter

While xml4h can work with multiple underlying XML libraries, some of these libraries are better (faster, more fully-featured) than others so it would be smart to use the best of the libraries available.

xml4h does exactly that: unless you explicitly choose an adapter (see below) xml4h will find the supported libraries in the Python environment and choose the “best” adapter for you in the circumstances.

Here is the list of libraries xml4h will choose from, best to least-best:

  • lxml
  • (c)ElementTree
  • ElementTree
  • minidom

The xml4h.best_adapter attribute stores the adapter class that xml4h considers to be the best.

Choose Your Own Adapter

By default, xml4h will choose an adapter and underlying XML library implementation that it considers the best available. However, in some cases you may need to have full control over which underlying implementation xml4h uses, perhaps because you will use features of the underlying XML implementation later on, or because you need the performance characteristics only available in a particular library.

For these situations it is possible to tell xml4h which adapter implementation, and therefore which underlying XML library, it should use.

To use a specific adapter implementation when parsing a document, or when creating a new document using the builder, simply provide the optional adapter keyword argument to the relevant method:

  • Parsing:

    >>> # Explicitly use the minidom adapter to parse a document
    >>> minidom_doc = xml4h.parse('tests/data/monty_python_films.xml',
    ...                           adapter=xml4h.XmlDomImplAdapter)
    >>> minidom_doc.root.impl_node  
    <DOM Element: MontyPythonFilms at ...
    
  • Building:

    >>> # Explicitly use the lxml adapter to build a document
    >>> lxml_b = xml4h.build('MyDoc', adapter=xml4h.LXMLAdapter)
    >>> lxml_b.root.impl_node  
    <Element {http://www.w3.org/2000/xmlns/}MyDoc at ...
    
  • Manipulating:

    >>> # Use xml4h with a cElementTree document object
    >>> import xml.etree.ElementTree as ET
    >>> et_doc = ET.parse('tests/data/monty_python_films.xml')
    >>> et_doc  
    <xml.etree.ElementTree.ElementTree object ...
    >>> doc = xml4h.cElementTreeAdapter.wrap_document(et_doc)
    >>> doc.root
    <xml4h.nodes.Element: "MontyPythonFilms">
    

Check Feature Support

Because not all underlying XML libraries support all the features exposed by xml4h, the library includes a simple mechanism to check whether a given feature is available in the current Python environment or with the current adapter.

To check for feature support call the has_feature() method on a document node, or has_feature() on an adapter class.

List of features that are not available in all adapters:

  • xpath - Can perform XPath queries using the xpath() method.
  • More to come later, probably...

For example, here is how you would test for XPath support in the minidom adapter, which doesn’t include it:

>>> minidom_doc.root.has_feature('xpath')
False

If you forget to check for a feature and use it anyway, you will get a FeatureUnavailableException:

>>> try:
...     minidom_doc.root.xpath('//*')
... except Exception, e:
...     e
FeatureUnavailableException('xpath',)

Adapter & Implementation Quirks

Although xml4h aims to provide a seamless abstraction over underlying XML library implementations this isn’t always possible, or is only possible by performing lots of extra work that affects performance. This section describes some implementation-specific quirks or differences you may encounter.

LXMLAdapter - lxml

  • lxml does not have full support for CDATA nodes, which devolve into plain text node values when written (by xml4h or by lxml‘s writer).
  • Namespaces defined by adding xmlns element attributes are not properly represented in the underlying implementation due to the lxml library’s immutable nsmap namespace map. Such namespaces are written correcly by the xml4h writer, but to avoid quirks it is best to specify namespace when creating nodes by setting the ns_uri keyword attribute.
  • When xml4h writes lxml-based documents with namespaces, some node tag names may have unnecessary namespace prefix aliases.

(c)ElementTreeAdapter - ElementTree

  • Only the versions of (c)ElementTree included with Python version 2.7 and later are supported.
  • ElementTree supports only a very limited subset of XPath for querying, so although the has_feature('xpath') check returns True don’t expect to get the full power of XPath when you use this adapter.
  • ElementTree does not have full support for CDATA nodes, which devolve into plain text node values when written (by xml4h or by ElementTree‘s writer).
  • Because ElementTree doesn’t retain information about a node’s parent, xml4h needs to build and maintain its own records of which nodes are parents of which children. This extra overhead might harm performance or memory usage.
  • ElementTree doesn’t normally remember explicit namespace definition directives when parsing a document. xml4h works around this when it is asked to parse XML data, but if you parse data outside of xml4h then use the library on the resultant document the namespace definitions will get messed up.

XmlImplAdapter - minidom

  • No support for performing XPath queries.
  • Slower than alternative C-based implementations.