Re: [xml] XPath evaluation and namespace prefixes

[Daniel Veillard <veillard redhat com>]

On Thu, Mar 04, 2004 at 12:22:00PM -0800, John L. Clark wrote:
> I am trying to evaluate an XPath expression that involves namespace
> prefixes.  Just trying to use node.xpathEval('//pre:element') fails
> with:
> 
>     Undefined namespace prefix
>     xmlXPathEval: evaluation failed
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in ?
>       File "/usr/lib/python2.3/site-packages/libxml2.py", line 361, in xpathEval
>         res = ctxt.xpathEval(expr)
>       File "/usr/lib/python2.3/site-packages/libxml2.py", line 6221, in xpathEval
>         if ret is None:raise xpathError('xmlXPathEval() failed')
>     libxml2.xpathError: xmlXPathEval() failed
> 
> And so I assumed that perhaps the XPath evaluation engine does not pick
> up on namespaces that are in scope, and that I would have to craft an

  in scope of what ??? no there is no definition for this...

> XPath evaluation context to specify my namespace prefix bindings.

  yes.

>     Note: I don't know if my assumptions are correct; it is hard to find
>           precise documentation on the semantics of the various API
>           points of entry, and even harder when it comes to the Python
>           API.  As such, corrections of any of my erroneous
>           assumptions/approaches are welcome.

  The ultimate is the XPath REC
    http://www.w3.org/TR/xpath
    Introduction:
"Expression evaluation occurs with respect to a context. XSLT and XPointer specify how the context is determined for XPath expressions used in XSLT and XPointer respectively. The context consists of:

* a node (the context node)
* a pair of non-zero positive integers (the context position and the context size)
* a set of variable bindings
* a function library
* the set of namespace declarations in scope for the expression
"

  Which clearly means "it's not XPath business to initialize that context"


> So I get started on experimenting with the xpathContext class in Python
> from the command line:
> 
>     Python 2.3.2 (#1, Nov 21 2003, 14:36:46)
>     [GCC 3.3.2] on linux2
>     Type "help", "copyright", "credits" or "license" for more information.
>     >>> import libxml2
>     >>> doc = libxml2.parseFile('overview.xml.rdf')
>     >>> xc = libxml2.xpathContext()
>     >>> xc
>     <libxml2.xpathContext instance at 0x40a4f30c>
>     >>> xc.setContextDoc(doc)
>     Segmentation fault 

  Might be a bug in Python bindings.

> The same thing happens when I try to use setContextNode (with a node
> instead of doc).  I am ...

  Setting the node and the document for the context node is anyway 
independant of setting the namespace declarations in scope for the expression

> Can someone point me in the right direction? 

  In any doubt readingt the spec first helps disambiguating.

> Is this latter problem a bug?

  Probably.

Daniel

-- 
Daniel Veillard      | Red Hat Network https://rhn.redhat.com/
veillard redhat com  | libxml GNOME XML XSLT toolkit  http://xmlsoft.org/
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