[PyQt] Python property in QObject masks AttributeErrors

[Benjamin Kloster]

On Tue, 2011-04-19 at 08:08 +0100, Phil Thompson wrote:
> On Tue, 19 Apr 2011 08:46:13 +0200, Benjamin Kloster
> <benjamin.kloster at videlco.eu> wrote:
> > 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.
> 
> The behaviour *is* the standard Python behaviour - it's not a PyQt thing.
> 
> Change "QtCore.QObject" with "object" and add a __getattr__ method that
> raises an exception and you'll see.
> 
> Phil
> 

Right. Sorry, my bad for not reading your first response correctly.
Anyway, I hope some poor souls will find this thread via Google when
they run into the same surprise.