ElementTree Namespace Prefixes

[Chris Spencer]

Does anyone know how to make ElementTree preserve namespace prefixes in
parsed xml files? The default behavior is to strip a document of all
prefixes and then replace them autogenerated prefixes like ns0, ns1,
etc. The correct behavior should be to write the file in the form that
it was read, which it seems to do correctly for everything except
namespace prefixes. The docs mention "proper" output can be achieved by
using the Qname object, but they don't go into any detail. Any help is
appreciated.

Thanks,
Chris Spencer

 

[Andrew Dalke]

Does anyone know how to make ElementTree preserve namespace prefixes in
parsed xml files?

See the recent c.l.python thread titled "ElemenTree and namespaces"
and started "May 16 2:03pm". One archive is at

http://groups-beta.google.com/group...b2e9f4a8f7338c/363f46513fb8de04?&rnum=3&hl=en

Andrew
(e-mail address removed)

 

[Chris Spencer]

See the recent c.l.python thread titled "ElemenTree and namespaces"
and started "May 16 2:03pm". One archive is at

http://groups-beta.google.com/group...b2e9f4a8f7338c/363f46513fb8de04?&rnum=3&hl=en

Thanks, although that thread didn't seem to resolve the issue. All the
first few links talk about is how to hack your own parser to make sense
of the Clark notation.

The problem at hand is with how Elementtree outputs namespaces and
represents the tag name in memory.

Given xml with no namespaces, Elementtree works perfectly. However, if
you give the root tag an xmlns attribute, Elementtree relabels all child
nodes with it's own prefix, completely defeating the purpose of the
default namespace. In my opinion, this is unacceptable behavior.

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.
With Elementtree this isn't so. Lundh apparently believes he knows
better than you and I on how our namespaces should be represented.

It's a shame the default ns behavior in Elementtree is in such a poort
staten. I'm surprised no one's forked Elementtree solely to fix this issue.

Anyways, Python's native minidom works as expected, so I'll probably use
that instead, even if the api is slightly less intuitive.

Chris

 

[Jarek Zgoda]

Chris Spencer napisa³(a):

Given xml with no namespaces, Elementtree works perfectly. However, if
you give the root tag an xmlns attribute, Elementtree relabels all child
nodes with it's own prefix, completely defeating the purpose of the
default namespace. In my opinion, this is unacceptable behavior.

There is no functional difference between default namespace and "normal"
namespace. Replacing "default" with "normal" has no effect for document
processing (namespace doesn't change, only prefix), although it looks
differently for humans. Anyway, XML is for machines, not for humans.

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.
With Elementtree this isn't so. Lundh apparently believes he knows
better than you and I on how our namespaces should be represented.

No, this is perfectly valid behaviour. Go, see spec.

It's a shame the default ns behavior in Elementtree is in such a poort
staten. I'm surprised no one's forked Elementtree solely to fix this issue.

There is at least one ElementTree API implementation that retains
prefixes, lxml.ETree. Go google for it.

 

[Fredrik Lundh]

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?

With Elementtree this isn't so. Lundh apparently believes he knows
better than you and I on how our namespaces should be represented.

do you even understand how XML namespaces work?

</F>

 

[Oren Tirosh]

says who?

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

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
representation?

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.

http://www.w3.org/TR/xml-c14n
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)

 

[Fredrik Lundh]

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
representation?

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.

http://www.w3.org/TR/xml-c14n
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.

you forgot

http://effbot.org/zone/element-infoset.htm

which describes the 3-node XML infoset subset used by ElementTree.

</F>

 

[Oren Tirosh]

you forgot

http://effbot.org/zone/element-infoset.htm

which describes the 3-node XML infoset subset used by ElementTree.

No, I did not forget your infoset subset. I was comparing it with other
infoset subsets described in various XML specifications.

I agree 100% that prefixes were not *supposed* to be part of the
document's meaning back when the XML namespace specification was
written, but later specifications broke that.

Please take a look at http://www.w3.org/TR/xml-c14n#NoNSPrefixRewriting

"... there now exist a number of contexts in which namespace prefixes
can impart information value in an XML document..."

"...Moreover, it is possible to prove that namespace rewriting is
harmful, rather than simply ineffective."

 

[Martijn Faassen]

Jarek Zgoda wrote:
[snip]

There is at least one ElementTree API implementation that retains
prefixes, lxml.ETree. Go google for it.

Just to make it explicitly clear, lxml is not a fork of ElementTree
fork, but a reimplementation of the API on top of libxml2.

ElementTree indeed retains prefixes, and since version 0.7 released
earlier this way, it's also possible to get some control over generation
of prefixes during element construction.

You can find it here:

http://codespeak.net/lxml

Regards,

Martijn

 

[uche.ogbuji]

Chris Spencer:
"""

Chris Spencer wrote:

says who?

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

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
representation?
"""

One can point out the XML namespaces spec all one wants, but it doesn't
matter. The fact is that regardless of what that spec says, as you
say, Chris, there are too many XML technologies that require prefix
retention. As a simple example, XPath and XSLT, W3C specs just like
XMLNS, uses qnames in context, which requires prefix retention.
Besides all that, prefix retention is generally more user friendly in
round-trip or poartial round-trip scenarios.

That's why cDomlette, part of 4Suite [1] and Amara [2], a more Pythonic
API for this, both support prefix retention by default.

[1] http://4suite.org
[2] http://uche.ogbuji.net/tech/4Suite/amara/