[PyQt] Bug report: multiple QApplication instances cause a segfault

Algis Kabaila akabaila at pcug.org.au
Thu Jun 23 05:14:07 BST 2011

On Thu, 23 Jun 2011 08:17:04 AM Baz Walter wrote:
> On 22/06/11 20:49, David Townshend wrote:
> > On Wed, Jun 22, 2011 at 2:34 PM, Baz Walter<bazwal at ftml.net>  wrote:
> >> according to the qt docs, only one application object should be created
> >> at a time, and there is a global qApp pointer which refers to the
> >> current instance.
> >> 
> >> this suggests that, in pyqt, something like the following should work:
> >> 
> >> from PyQt4 import QtGui
> >> QtGui.qApp = app = QtGui.QApplication([])
> >> app.quit()
> >> QtGui.qApp = app = None
> [snip]
> > I've investigated further and found that its caused by the gtk style
> > which I am using, since I am running gnome. Simply changing the qt style
> > avoids the problem. I recall reading about similar issues before with
> > the gtk-qt style, so I think that this is a know problem.  However, I
> > noticed that after changing the style, python still segfaults on exit. 
> > gdb indicates that it is something in the QApplication destructor, and
> > it seems that deleting the
> > 
> > instance after quitting solves this, e.g.:
> >>>> from PyQt4.QtGui import QApplication
> > 
> >>>> for i in range(10):
> > ...     app = QApplication([])
> > ...     app.quit()
> > ...     del app
> > ...
> > 
> >>>> exit()
> > 
> > I experimented a bit with setting qApp = None, but it didn't make a
> > difference.
> my point was simply that you need to make sure you remove all references
> to the current qpplication instance before creating a new one.
> in general, setting qApp to the instance of qapplication isn't
> necessary. but it's worth noting that qApp won't refer to the same
> *pyqt* object unless it's explicitly over-written. this may become
> relevant when working with subclasses of qapplication. of course, if you
> don't use qApp, it won't make any difference, as you say ;-)


This does not explain why the IDLE quits so early in the test, even before CR 
is pressed... Actually, in python3.2 if one programs in IDLE, one finds that 
IDLE is useless with PyQt as it quits (just vanishes from the screen...) if 
one tries to run a script after modifying it.  Basically, IDLE works only if 
it is invoked from scratch each time one wants to run a program.

I suggest that because of such misbehaviour it is not useful to look for 
faults in the test case - there is a fault, either in Python3 IDLE, Python 3, 
PyQt, Qt itself.

As for the PyQt bug tracker - I was not able to find it, though I saw bug 
reports in several places, though I said earlier that I did see it somewhere.  

I would write it down to my aging memory....


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