Re: [xml] Support for Python

[Dave Kuhlman <dkuhlman cutter rexx com>]

On Mon, Jan 28, 2002 at 02:32:38PM -0500, Daniel Veillard wrote:
> 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 :

I think the trick is to add the flag "-shadow" when you run swig. 
Here is the command line that I use:

swig -python -shadow -I/w2/XML/Libxml/libxml2-2.4.13/include/libxml pydom.i

And here is my pydom.i:

============================================================

%module pydom
 
 
%{
#include "xmlmemory.h"
#include "parser.h"
#include "parserInternals.h"
#include "tree.h"
xmlChar * c2x(char * s1)
{
    return (xmlChar *)s1;
}
char * x2c(xmlChar * s1)
{
    return (char *)s1;
}
%}
 
xmlChar * c2x(char * s1);
char * x2c(xmlChar * s1);
 
%include "tree.h"
%include "parser.h"

============================================================


> > > > 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

OK. I don't understand this either.  I'm using Python 2.2, but I do
not believe there is anything in it that requires version 2.2.

I tried it with Python 2.1 (after rebuilding the .so with Python
2.1 include files).  My tree walk test runs OK.

>   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.

SWIG does some "name mangling" (at least when I generate shadow
classes) to keep methods in separate classes unique.  For the
xmlDoc, in my generated code, I believe the function is called
xmlDoc_version_get.


> > > 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.

Sounds to me that you are saying that SWIG is worth exploring a bit
further.  I agree with that.

> > 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.

Another good reason for using shadow classes.  We'll have to check
it, but I believe that SWIG generates reasonably conservative code.

  - Dave

> > > 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/
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-- 
Dave Kuhlman
dkuhlman rexx com
http://www.rexx.com/~dkuhlman