[PyQt] QWidget not deleted with its parent window

[Tavian Barnes]

On 4 November 2011 00:17, Phil Thompson <phil at riverbankcomputing.com> wrote:
> On Thu, 3 Nov 2011 19:49:03 -0400, Tavian Barnes
> <tavianator at tavianator.com> wrote:
>> I originally posted this on stackoverflow[1], where I learned that
>> this seems to be a bug in PyQt 4.8.6.  (I'm using Qt 4.7.4 if that's
>> relevant.)
>>
>> Basically, the __del__ method of the central widget of a window is
>> never called.  Example:
>>
>> #!/usr/bin/env python
>>
>> from PyQt4 import QtGui
>>
>> class Preview(QtGui.QWidget):
>>   def __init__(self, parent):
>>     QtGui.QWidget.__init__(self, parent)
>>
>>   def __del__(self):
>>     print("Deleting Preview")
>>
>> class PreviewWindow(QtGui.QMainWindow):
>>   def __init__(self):
>>     QtGui.QMainWindow.__init__(self)
>>
>>     self.widget = Preview(self)
>>     self.setCentralWidget(self.widget)
>>
>>   def __del__(self):
>>     print("Deleting PreviewWindow")
>>
>> if __name__ == "__main__":
>>   app = QtGui.QApplication(["Dimension Preview"])
>>   window = PreviewWindow()
>>   window.show()
>>   app.exec()
>>   # del window
>>
>> If I uncomment the "del window" line, "Deleting Preview" prints as
>> expected.  Otherwise, I just see "Deleting PreviewWindow".  Apparently
>> pyqt 4.8.5 works correctly.
>>
>> [1]
>>
> http://stackoverflow.com/questions/7988578/qwidget-not-deleted-with-parent-window
>
> It's not a bug, just a (subtle) change in (undefined) behaviour. Without
> the explicit "del window" the objects are being destroyed as part of the
> interpreter shutdown and in an unpredictable order. This is the common
> cause of crashes on program exit. To help avoid this SIP v4.13 (it's not
> PyQt) prevents any Python reimplementations of C++ methods (and also
> __del__) from being called once the interpreter starts to shutdown. The
> surprise, if any, is why PreviewWindow.__del__ is being called.
>
> Basically any program that relies on specific behaviour after the
> interpreter starts to shutdown is buggy. Also I would never use __del__
> anyway because of its impact on the cyclic garbage collector.
>
> Phil
>

Ah okay, so is it recommended to manually delete it with "del
window"then?  The real-life case is that Preview holds a Python
reference toan object that wraps a C type, and since it's never
destroyed, neitheris the C object.  The underlying C library then
detects a memory leakand spews an ugly warning when the application
exits.
-- 
Tavian Barnes