lxml upcoming new features

lxml upcoming new features

lxml has undergone quite a bit of development since lxml 0.6. While 0.7 is not yet released, this release should be coming soon, and to whet your appetites here's a partial list of new features:

• XMLSchema validator support

• XInclude support

• more control over namespace prefixes when generating XML

I'll talk about the least spectacular sounding feature that in fact cost me the most time to implement: control over namespace prefixes.

I found myself generating XML quite a lot in a recent project, and experimented with a bunch of different APIs in lxml to support which prefixes are created for namespaces (and what the default namespace should be).

As you may or may not know, the ElementTree API doesn't have any official support for controlling what prefixes are outputted. This can result in entirely correct but ugly XML with namespace prefixes like ns0, ns17, etc.

Even though prefixes are not part of the XML infoset, some control over what they look like in XML is frequently desirable, as the intent of XML is to be at least somewhat human readable. It's easy to start leaning too much into the other direction, though: one should be careful not to offer too much control to the user either.

The W3C DOM, as usual, offers way too much API for namespace handling, which results in all kinds of scary interactions I don't want to worry about. I did add an attribute to read prefix information, but unlike the DOM, will not make this writeable, as this quickly gets pretty insane, so that route towards namespace control is out.

After quite a bit of thinking, I ended up supporting a second special argument to the Element and SubElement constructors. The first special argument, part of ElementTree, is 'attrib', which is a dictionary to control attributes. I added a new argument called 'nsmap', which is a dictionary to control namespaces. The keys are the namespace prefixes, the values the namespace URIs. A key of 'None' means set the default namespace. If a namespace is already known higher up in the tree, that will be reused instead.

Here's an example:

>>> from lxml import etree
>>> e = etree.Element('{http://ns.infrae.com/foo}bar',
>>> ... nsmap={'foo': 'http://ns.infrae.com/foo'})
>>> e.prefix
'foo'