[PyQt] Exception within Slot causes Abort

Florian Bruhin me at the-compiler.org
Sun Mar 20 15:46:41 GMT 2016

* Dave Gradwell <davegradwell at yahoo.co.uk> [2016-03-20 13:20:41 +0000]:
> With PyQt 5.5 the behaviour is inconsistent (consequence depends on context);
> and aborting because you encountered something exceptional seems really quite
> disproportionate to me. And not at all in the spirit of Exceptions as a
> programmer's friend.

I'm not aware of any programming language which has exceptions and
does not abort on an unhandled one.

> > No, it doesn't continue. It quits.
> I am unable to see this behaviour.
> For me, with PyQt 5.4, it continues. 
> And to clarify what I mean by 'it continues' I could either say 'it
> doesn't abort' or 'it behaves like a non PyQt Python program would'.
> I am not suggesting it pushes on past the exception or something
> ridiculous like that.

I still don't follow. Any Python code not involving PyQt does *quit*
when there's an unhandled exception:

	$ cat exc.py
	print("Before an exception")
	print("After an exception (not printed)")

	$ python exc.py
	Before an exception
	Traceback (most recent call last):
	  File "exc.py", line 2, in <module>
	ZeroDivisionError: division by zero

Since PyQt 5.5, PyQt does the same.

If you handle the exception, neither a PyQt nor a non-PyQt application
quits - adopted from your example:

    $ cat exc2.py
    import sys
    from PyQt5 import QtCore, QtWidgets

    def timerFinished():
        print("timer finished")
            raise Exception
        except Exception:

    app = QtWidgets.QApplication(sys.argv)

    timer = QtCore.QTimer()


    $ python exc2.py
    timer finished
    timer finished

So really the only thing which changes is that PyQt 5.5+ does not
silently ignore unhandled exceptions anymore, like *everything else
does*. How is this not consistent?

> If so, what construct can I use to regain the nice behaviour of PyQt 5.4?

You can do:

    sys.excepthook = traceback.print_exception

But again, you'll have undefined behaviour on exceptions inside C++
virtuals then.

> Also, the Abortion is in full swing by this time.  I.e. We're doomed.  No?

If you have an unhandled exception, you're kind of doomed anyways?


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