Re: [xml] Support for Python

[Daniel Veillard <veillard redhat com>]

On Mon, Jan 28, 2002 at 10:51:58AM -0800, Dave Kuhlman wrote:
> On Mon, Jan 28, 2002 at 02:41:48PM +0000, Gary Benson wrote:
> > It's a lot nicer defining the classes in Python than in C, and you have 
> > the added benefit that it is a lot easier for someone trying to use the 
> > library to see how it works.
>
> I should have been a bit more clear about this.  When you ask SWIG
> to generate "shadow" classes, which I did, SWIG generates a .py
> file with Python classes in it.  So when you do:
>
> > > > doc = p.xmlParseFile('test1.xml')
> > > > root = doc.children
>
> you have created an instance of the Python class xmlDoc and an
> instance of the Python class xmlNode, each of which hold a
> reference to an underlying C object (xmlDoc and xmlNode, defined in
> tree.h).

  I still haven't been able to get SWIG to work. It does write a c file,
but emits errors on fetaures it doesn't support:
parser.h : Line 236. Function pointers not currently supported (ignored).
parser.h : Line 520. Variable length arguments not supported (ignored).

  And doesn't seem to produce a .py . It produce pydom_wrap.c, which then
compiles and load but I'm unable to load anything :

> > > import pydom
> > > doc = pydom.xmlParseFile("test1.xml")
> > > print 'version: %s' % pydom.x2c(doc.version)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: version

  tried version_get which from my reading of the C code should work,
but it fails too ... weird. I have tried with both 1.5 and 2.1 without
success.

> > Also (but don't quote me on this) I'm not sure that you can derive
> > subclasses from classes defined in C modules. I was was unable to find a C
> > module that exports a class to test however.
>
> I agree with your point.  We need Python classes.  SWIG can
> generate them.  We have to decide whether to use them, whether and
> how to modify them, etc.

  My current appraoch is to try with SWIG, if it looks really nice
adopt it as is, if not keep the good points and write a generator
based on the interface definition and the SWIG output examples.

> Actually, in Python 2.2, with the new "unification of types and
> classes", it is possible to implement _types_ in C that can be
> subclassed.  But your point is still valid:  Real classes are
> better.

  Well I would really like to have this portable to both 1.5 and 2.1.

> > If we write the classes in Python then this is trivial -- call xmlFreeDoc 
> > in the XmlDocumentClass's destructor and let Python's garbage collector 
> > work out when to do it.
>
> Interesting idea.  Because of your suggestion, I looked at the
> generated code.  And, I can modify the SWIG generated code so that
> the Python __del__ method in the Doc class will call a C delete
> method and I can modify the C delete method so that it calls
> xmlFreeDoc.

  Really dangerous. As you pointed out too as soon as Python will loose
a pointer to the root it would then deallocate the full subtree. Too
dangerous to be allowed. I started toying with the idea but as soon
as I tried to build an example I realized it would really make a big problem.
  I don't think we can have garbage collectable xmlDoc, though nearly
every other classes exposed should be garbageable.

Daniel

-- 
Daniel Veillard      | Red Hat Network https://rhn.redhat.com/
veillard redhat com  | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
http://veillard.com/ | Rpmfind RPM search engine http://rpmfind.net/