[PyKDE] ref counting

Stefan Seefeld seefeld at sympatico.ca
Sat Feb 14 07:54:01 GMT 2004

Phil Thompson wrote:
> On Saturday 14 February 2004 04:46, Stefan Seefeld wrote:
>>hi there,
>>I'm experimenting with pyqt and I'm running in
>>some strange behavior.
>>The examples3/canvas/canvas.py demo contains
>>the following lines:
>>     def newView(self):
>>         self.m=Main(self.canvas,None,"new windiw",Qt.WDestructiveClose)
>>         qApp.setMainWidget(self.m)
>>         self.m.show()
>>         qApp.setMainWidget(None)
>>What I'm wondering about is the reason for the 'Main' instance to
>>be stored in an attribute. In fact, if I replace 'self.m' by 'm'
>>the applet doesn't work any more, i.e. as soon as I create the
>>second view the first will be destructed. What's the reason for
>>this ? It appears the ref count for the view drops to zero so
>>it gets destroyed...
> Correct, 'm' is garbage collected when newView() returns, 'self.m' is garbage 
> collected when 'self' is garbage collected.
>>But in the above, it would seem the second call to 'newView' would
>>free the first view, and thus destroy it, too. That, however, is
>>not the case.
> But the second call to newView() has a different self.

How so ? Is it not the 'self' of the view containing the menu I clicked on ?
A little try showed the same value for 'id(self)' if I call 'newView'
multiple times.

I guess I need to understand the 'setMainWidget' trick in order to see
why it works. 'm' is set to be the main widget, then it is mapped, then
'None' is set to be the main widget. What does this call do ? (I note
it's also in the C++ example, so it probably isn't related to python's
ref counting).

Thanks for your help !


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