[xml-bindings]Limitations of attribute nodes in Python

[Marc-Antoine Parent <maparent acm org>]

I was coding up an XUpdate implementation in Python, and I realized  
that, when a XPath gave me an attribute node, there was actually very  
little I could do with it.

A) For example, if I change the content, it will have no effect on the  
owning node.

>>> import sys
>>> from libxml2 import *
>>> xmlText="""<a b="x"/>"""
>>> doc = parseMemory(xmlText,len(xmlText))
>>> doc
<xmlDoc (None) object at 0x74468>
>>> rootElement=doc.getRootElement()
>>> rootElement
<xmlNode (a) object at 0x6d3a0>
>>> b=rootElement.get_properties()
>>> b
<xmlAttr (b) object at 0x255968>
>>> b.content
'x'
>>> b.content='y'
>>> b.content
'y'
>>> doc.dump(sys.stdout)
<?xml version="1.0"?>
<a b="x"/>
33

B) So I thought I would go back to the parent to do a setProp, which is  
slightly hassly, but then I realized I do not have an easy access to  
the parent!
>>> b.parent
>>> b.get_parent()
>>>

Of course, I can do this, which fortunately works, but is somewhat  
heavy-handed:

>>> b.xpathEval("..")
[<xmlNode (a) object at 0x255a08>]

C) I tried other ways to obtain the parent; some failed, and some  
worked: I do not pretend I always knew what I was doing, there, and I  
am sure some of this is not expected to work, but the bus error in my  
final attempt still came as a slight shock.

>>> c = doc.xpathNewContext()
>>> c.setContextNode(b)
>>> c.xpathEval("..")
[<xmlNode (a) object at 0x74fd0>]
>>> b.xpathNextParent(c)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File  
"/System/Library/Frameworks/Python.framework/Versions/2.3/lib/ 
python2.3/site-packages/libxml2.py", line 3455, in xpathNextParent
    if ret is None:raise xpathError('xmlXPathNextParent() failed')
libxml2.xpathError: xmlXPathNextParent() failed
>>> b.xpathEval("..")
[<xmlNode (a) object at 0x255a08>]
>>> c.setContextNode(b)
>>> pc=c.xpathNewParserContext("")
>>> pc.xpathNextParent(b)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File  
"/System/Library/Frameworks/Python.framework/Versions/2.3/lib/ 
python2.3/site-packages/libxml2.py", line 5501, in xpathNextParent
    if ret is None:raise xpathError('xmlXPathNextParent() failed')
libxml2.xpathError: xmlXPathNextParent() failed
>>> pc.xpathNextAncestor(b)
<xmlNode (a) object at 0x74ee0>
>>> b.xpathNextAncestor(c)
Bus error

I settled on pc.xpathNextAncestor(b) as plausibly the most efficient.

Please tell me if you want me to bugzilla A, B, C separately or as a  
block. Also, if some of them do feel normal to you, I would like to  
understand why.

Marc-Antoine Parent