Re: [xml] Xpath issues with libxml2

[Daniel Veillard <veillard redhat com>]

On Thu, Feb 12, 2015 at 11:36:12AM -0500, Alex Boese wrote:
> I kinda solved my own problem. I'm posting an example of what works, since I had difficulty establishing 
> this:
>
> import libxml2
> import sys
>
> XML1 = """<f:Foo xmlns:f="http://www.w3.org/f#";><b:Bar
> xmlns:b="http://www.w3c.org/b#";>foobar</b:Bar></f:Foo>"""
>
> xml_parser_options = libxml2.XML_PARSE_RECOVER + libxml2.XML_PARSE_NONET
> size = sys.getsizeof(XML1)
> doc = libxml2.readMemory(XML1,size,None,'UTF-8',xml_parser_options)
>
> context = doc.xpathNewContext()
> #context.xpathRegsterNs('f',' http://www.w3.org/f#')
> #context.xpathRegsterNs('b',' http://www.w3.org/b#')
>
> print "test 1"
> res = context.xpathEval('/*[local-name()="Foo"]/*[local-name()="Bar"]')
> i = 0
> for node in res:
>     print i,':',node
>     i = i + 1
>
> #print "test 2"
> #res = context.xpathEval('/f:Foo/b:Bar')
> #i = 0
> #for node in res:
> #     print i,':',node
> #     i = i + 1
>
> Result:
> Test 1
> 0 : <b:Bar xmlns:b="http://www.w3c.org/b#";>foobar</b:Bar>
>
> Of course, if you comment in the commented out portions you get test 2 which
> returns the exact same thing. The point I'm making above is that it might not even be a requirement to 
> register each namespace! It depends really on what kind of xpath you're using.
>
> Of course, feel free to bash my answer and tell me I'm wrong.

  You only need to register in the XPathcontext the namespaces that
you are using in the XPath queries related to this context. Also what
prefix you use is of no matter, XPath will to the matching on the
namespace name (the URL for the namespace), never the prefix,

Daniel
> -A
>
> > > I'm thinking it should look something like this, assuming Python language:
> > >
> > > Import xmlsec
> > > Import libxml2
> > >
> > > ...
> > >
> > >   def test_get_xml_fragment(self,xpath,ns):
> > >        ret = None
> > >        context = self.doc.xpathNewContext()
> > >        if ns is None:
> > >
> > > context.xpathRegisterNs(ns,'http://127.0.0.1/#no_place_like_home')
> > >        res = context.xpathEval(xpath)
> > >        i = 0
> > >        for node in res:
> > >             print i,':',node
> > >             i = i + 1
> > >
> > >
> > > Please note there is an object in the mix, and I can get the whole to work great without namespaces. One 
> > > good example of how Python should be handling namespaces in this case would be great. Even telling me 
> > > it's forever broken...this too would be good to know. (Also, this is not an ideal example...I get this. 
> > > I want something that hobbles before I get something hat soars.)
> > >
> > > Thanks in advance.
> > >
> > > Sent from my Planet
> > > > Message: 2
> > > > Date: Fri, 30 Jan 2015 09:03:40 -0600
> > > > From: Ross Reedstrom <reedstrm rice edu>
> > > > To: xml gnome org
> > > > Subject: Re: [xml] Xpath issues with libxml2
> > > > Message-ID: <20150130150340 GA28262 rice edu>
> > > > Content-Type: text/plain; charset=us-ascii
> > > >
> > > > Alex -
> > > > With out examples of what you've tried, it's hard to diagnose the problem.
> > > > However, seeing 'namespaces' and 'never returns anything' makes me think you're
> > > > having issues with the default namespace concept. While XML documents have a
> > > > default namespace, XPaths do not. Once you use namespaces in an xml document,
> > > > all your xpaths will need to use namespace declarations for all the path parts,
> > > > even for tags that are defaulted in the document. So, you'll need to declare
> > > > a namespace prefix that matches the default namespace in the doc. Give us
> > > > a small example that you've tried that doesn't work, we'll fix it.
> > > >
> > > > Ross
> > > >
> > > > > On Fri, Jan 30, 2015 at 08:44:15AM -0500, Alex Boese wrote:
> > > > > Forgive me if this is a deprecated approach (as I am not fully aware), but I was utilizing "default" 
> > > > > libxml2 bindings (not lxml) in Python to retrieve xml fragments via xpath functions. Normally this 
> > > > > seems to work fine, but with namespace declarations this seems especially problematic as nothing ever 
> > > > > returns and nothing errors. Would it be possible to confirm what correct functions and order of 
> > > > > operations would be for this? Even if the example is C, I can translate that to Python.
> > > >
> > > > -- 
> > > > Ross Reedstrom, Ph.D.                                 reedstrm rice edu
> > > > Systems Engineer & Admin, Research Scientist        phone: 713-348-6166
> > > > Connexions                  http://cnx.org            fax: 713-348-3665
> > > > Rice University MS-375, Houston, TX 77005
> > > > GPG Key fingerprint = F023 82C8 9B0E 2CC6 0D8E  F888 D3AE 810E 88F0 BEDE
> > > >
> > > >
> > > > ------------------------------
> > > >
> > > > Subject: Digest Footer
> > > >
> > > > _______________________________________________
> > > > xml mailing list
> > > > xml gnome org
> > > > https://mail.gnome.org/mailman/listinfo/xml
> > > >
> > > >
> > > > ------------------------------
> > > >
> > > > End of xml Digest, Vol 128, Issue 2
> > > > ***********************************
> > > -------------- next part --------------
> > > An HTML attachment was scrubbed...
> > > URL: <https://mail.gnome.org/archives/xml/attachments/20150210/13669f43/attachment.html>
> > >
> > > ------------------------------
> > >
> > > Subject: Digest Footer
> > >
> > > _______________________________________________
> > > xml mailing list
> > > xml gnome org
> > > https://mail.gnome.org/mailman/listinfo/xml
> > >
> > >
> > > ------------------------------
> > >
> > > End of xml Digest, Vol 129, Issue 1
> > > ***********************************

> _______________________________________________
> xml mailing list, project page  http://xmlsoft.org/
> xml gnome org
> https://mail.gnome.org/mailman/listinfo/xml


-- 
Daniel Veillard      | Open Source and Standards, Red Hat
veillard redhat com  | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
http://veillard.com/ | virtualization library  http://libvirt.org/