[PyKDE] KHTML method returning null

Paul Giannaros ceruleanblaze at gmail.com
Tue Mar 6 15:30:15 GMT 2007

Ah, problem solved!

On Tuesday 06 March 2007 01:00, Jim Bublitz wrote:
> On Monday 05 March 2007 08:29, Paul Giannaros wrote:
> > The documentation for KHTML's DOM::Document::getElementById (
> > http://api.kde.org/3.5-api/kdelibs-apidocs/khtml/html/classDOM_1_1Documen
> >t. html#a20 ) show's that the method returns null when an element in the
> > webpage by the given ID hasn't been found. I would have thought that
> > would have mapped to Python's None, but instead I'm getting a
> > khtml.DOM.Element.
> If the underlying C++ call returns null, the PyKDE call should return None
> (see sip_api_return_from_new_instance in siplib/siplib.c)

Aha, I see.

> > Whenever I try to call a method on it, my application crashes with
> > "terminate called after throwing an instance of 'DOM::DOMException'".
> > Firstly, is this intended behaviour?
> I don't think so.
> > Secondly, is there any way that I can
> > check if the return value from that call is a valid object?
> Try accessing some method of the object, I suppose (you can also check its
> Python type, of course, but that only tells you what Python thinks the
> object is).
> I'd suggest posting a short example program that demonstrates the problem
> (include the html necessary to cause it too).
> Most of this stuff is straightforward sip wrappers - meaning sip generates
> the code the same as for any other C++ class, and there isn't anything
> unusual or customized that I can see. That suggests the most likely problem
> is in the implementation you're doing - the commonest problem is not
> keeping a Python reference to an object you're trying to access, for
> example.
> However the khtml, and especially DOM, stuff hasn't gotten a lot of use,
> and I have no application code to test it with, so problems either in PyKDE
> or the underlying KDE code are quite possible too.
> Either way, an example would be useful to see what's happening.
> Jim

It turns out that it was a documentation / cognitive problem. The docs say it 
returns null, but it turns out it just returns a 'null element'. You can call 
isNull() on the returned object to check whether or not it's valid. 


> > Maybe there is
> > something you can do with the sip module? (though I can't see anything in
> > the docs)
> >
> > Thanks,
> > Paul
> >
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