[PyQt] Python property in QObject masks AttributeErrors

[Benjamin Kloster]

On Mon, 2011-04-18 at 22:18 +0100, Phil Thompson wrote:
> On Mon, 18 Apr 2011 14:07:13 +0200, Benjamin Kloster
> <benjamin.kloster at videlco.eu> wrote:
> > Hi everyone,
> > twice now I've stumbled over a subtle bug when subclassing QObject. When
> > the getter of a python property raises an AttributeError for any
> > attribute or object, PyQt seems to reraise this error improperly,
> > clearing the traceback and often reporting a false object type and
> > attribute name. See the attached script for a minimal example.
> > 
> > Regards,
> > Ben
> 
> It's working as it should.
> 
> If the getter raises an AttributeError Python then calls any __getattr__
> method. If there is no method then the exception raised by the getter is
> propagated. If there is a __getattr__ then the getter exception is
> discarded and the exception raised by __getattr__ is propagated.
> 
> QObject implements __getattr__ so the exception raised by the failure of
> QtCore.iDontExist is discarded. The exception that QObject.__getattr__
> raises is what you are seeing.
> 
> Phil
> 

The normal behavior of a Python object is to report the first
AttributeError ("'module' object has no attribute 'iDontExist'). At
least in Python 2.6, 2.7 and 3.2 it is. Just replace "QtCore.QObject"
with "object" in the example to reproduce this.

Especially if a getter is rather involved and maybe even calls third
party libraries that produce the original AttributeError, debugging
becomes confusing at best with PyQt's behavior.

If it's not feasible to implement the correct (i.e. standard Python)
error reporting, I can live with that. However, I couldn't find any note
of this caveat in PyQt's documentation. So if it's really not there, it
should probably be added.

Ben