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List:       pykde
Subject:    Re: [PyKDE] PyQt4 Questions
From:       Phil Thompson <phil () riverbankcomputing ! co ! uk>
Date:       2006-03-20 9:47:15
Message-ID: 200603200947.15071.phil () riverbankcomputing ! co ! uk
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On Saturday 18 March 2006 4:11 pm, Detlev Offenbach wrote:
> Am Samstag, 18. März 2006 16:29 schrieb Giovanni Bajo:
> > Detlev Offenbach <detlev@die-offenbachs.de> wrote:
> > > I have a dialog, that creates a new dialog and shows it. The code is
> > > like
> > >
> > > self.dlg = MyDialog(self)
> > > dlg.show()
> >
> > MyDialog is a QObject, and it is constructed as child of self. Its
> > lifetime becomes bound to the lifetime of "self", pretty much like every
> > other child of self (including labels, buttons, etc.).
> >
> > > This code is part of a slot. Whenever this code is hit, a new dialog is
> > > created with destroying the old one. I thought, that the garbage
>
> Typo: created without destroying ...
>
> > > collector should take care of deleting the old one.
> >
> > No, because like any other QObject, its lifetime is bound to its parent,
> > *irrespective* of the lifetime of the Python object (which can be
> > destroyed and recreated on demand). Think of how many widgets you
> > (probably) create to compose a dialog and for which you do *not* keep a
> > Python reference.
>
> In the meantime I did some more tests. I changed the QDialog derived dialog
> to a QWidget derived one and now it get deleted by the garbage collector,
> when I create a new instance. It seems, that the C++ QDialog gets detached
> from its Python proxy.
>
> > > Same thing happens with nonmodal dialogs spawned from the QMainWidget.
> >
> > When
> >
> > > closing the application, I have to close every single dialog, that was
> > > displayed via the show() method, individually.
> >
> > The point is that PyQt *specifically* implements a workaround for QDialog
> > and QPopupMenu when used as *modal* dialogs. If you run the exec_loop()
> > method (and thus request modal behaviour), their lifetime semantic is
> > explicitally changed and becomes bound to the *Python* reference. This
> > allows to write Python code to use modal dialogs which works
> > "out-of-the-box".
> >
> > For modeless dialogs, you're out of luck. There are at least a couple of
> > ways to do what you want:
> >
> > * Explicitally delete the widget before constructing the new one. There
> > is no equivalent of "delete dlg" in Python, so the common way is to call
> > the deleteLater() method. This is usually enough. Notice that you can
> > force the deleteLater() message to be immediately processed by doing:
> > qApp.sendPostedEvents(widget, QEvent.DeferredDelete). Usually you don't
> > need this, though, and waiting till the next event loop is good enough.
>
> It worked ok in PyQt3 but shows the weird (at least for me) behavior for
> PyQt4. Therefore I suspect something wrong with PyQt4 and QDialog.

Can you send me your test case. The PyQt3 and PyQt4 code to handle this is 
identical.

Phil

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