[PyKDE] problem with exceptions within events

Phil Thompson phil at riverbankcomputing.co.uk
Fri Feb 25 09:40:23 GMT 2005

> Hello,
> it seems that Python exceptions generated within event handlers do not get
> propagated correctly. We use a sys.excepthook to abort gracefully with a
> messagebox from our application, and this seems to work in most cases but
> not within event handlers. For instance:

The old (ie. pre-SIP 4.2) behaviour was to call PyErr_Print() if an
exception was raised in a Python re-implementation of a C++ virtual
method. I never really liked this as it meant that the exception couldn't
be caught with try/except.

In SIP 4.2 I removed the calls to PyErr_Print() and added a check for a
Python exception after each called to a C++ method. The theory being that
a try/except around the corresponding Python method call would catch any
exception raised. There turned out to be two problems with this...

- The implementation was buggy. The SIP module was using exceptions in the
meantime - it should have been saving and restoring any existing
exception. With this fixed exceptions are caught properly.

- It only works if the C++ method returns to allow the test for the
exception to be made. The one place where this doesn't happen is
QApplication.exec_loop(). Therefore you still have to rely on providing
your own sys.excepthook. However, it turns out that the excepthook is only
called from within PyErr_Print().

So, rather than try and deal with the two conflicting requirements (being
able to catch as many exceptions as possible, and being able to detect
exceptions in exec_loop()) I plan to simply revert to the pre-SIP 4.2
behaviour - unless anybody has any other bright ideas.

I will do this in the next couple of days, leave it for a week for
feedback, and release SIP 4.2.1.


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