ElementTree Namespace Prefixes

Oren Tirosh oren.tirosh at gmail.com
Mon Jun 13 18:33:25 CEST 2005

Fredrik Lundh wrote:
> Chris Spencer wrote:
> > If an XML parser reads in and then writes out a document without having
> > altered it, then the new document should be the same as the original.
> says who?

Good question. There is no One True Answer even within the XML

It all boils down to how you define "the same". Which parts of the XML
document are meaningful content that needs to be preserved and which
ones are mere encoding variations that may be omitted from the internal

Some relevant references which may be used as guidelines:

* http://www.w3.org/TR/xml-infoset
The XML infoset defines 11 types of information items including
document type declaration, notations and other features. It does not
appear to be suitable for a lightweight API like ElementTree.

* http://www.w3.org/TR/xpath-datamodel
The XPath data model uses a subset of the XML infoset with "only" seven
node types.

The canonical XML recommendation is meant to describe a process but it
also effectively defines a data model: anything preserved by the
canonicalization process is part of the model. Anything not preserved
is not part of the model.

In theory, this definition should be equivalent to the xpath data model
since canonical XML is defined in terms of the xpath data model. In
practice, the XPath data model defines properties not required for
producing canonical XML (e.g. unparsed entities associated with
document note). I like this alternative "black box" definition because
provides a simple touchstone for determining what is or isn't part of
the model.

I think it would be a good goal for ElementTree to aim for compliance
with the canonical XML data model. It's already quite close.

It's possible to use the canonical XML data model without being a
canonical XML processor but it would be nice if parse() followed by
write() actually passed the canonical XML test vectors. It's the
easiest way to demonstrate compliance conclusively.

So what changes are required to make ElementTree canonical?

1. PI nodes are already supported for output. Need an option to
preserve them on parsing
2. Comment nodes are already support for output. Need an option to
preserve them on parsing (canonical XML also defines a "no comments"
canonical form)
3. Preserve Comments and PIs outside the root element (store them as
children of the ElementTree object?)
4. Sorting of attributes by canonical order
5. Minor formatting and spacing issues in opening tags

oh, and one more thing...

6. preserve namespace prefixes ;-)
(see http://www.w3.org/TR/xml-c14n#NoNSPrefixRewriting for rationale)

More information about the Python-list mailing list