[PySide] Bug is worse (was: QObject.destroyed() is not emitted)

[Nathan Smith]

I am very interested in this because I have a PySide application where my
widgets don't die gracefully (and destroyed isn't signalling their death).
 I use properties heavily in my code, so I was excited when I saw your
email thinking they were the culprit.  However, I just tried your example
using Python 2.7.1 and PySide 1.1.1, and I can't reproduce the problem.  My
table prints "destroyed" immediately when I call del m.  Are you sure that
"property" creates a reference?

I did verify in my environment that onDestroy must be static to function
correctly.


On Thu, Nov 29, 2012 at 4:40 PM, Christian Tismer <tismer at stackless.com>wrote:

> Hi friends,
>
> On 11/29/12 6:37 PM, Stephan Deibel wrote:
> > Alexey Vihorev wrote:
> >> Thanks, nice find, but... I hit the next hurdle trying to go this
> >> way. The signal QObject.destroyed(obj) is passing no arguments
> >> (probably because obj is already destroyed), so my static method has
> >> nothing to work on. Which kind of devaluates the whole idea, IMHO.
> >> And in PyQt4 it*does* pass the object. Even more: in PyQt4 there is
> >> no need for static method approach, as it works perfectly with
> >> instance methods:
> >
> > Yea, you would have to bind the necessary data to the callback like this:
> >
> >   def on_destroy(val1=self.whatever, val2=self.something):
> >     print 'destroyed'
> >   self.destroyed.connect(on_destroy)
> >
> > Whether this is possible or useful in your case is of course going to
> > depend on the details of the code.
> >
> > Having 'destroyed' be emitted before the object is destroyed and
> > getting the object reference as an arg makes more sense to me. That is
> > what PyQt seems to do and it is what QObject does under Qt using C++,
> > so I'd call this a bug in PySide.  I've added
> > https://bugreports.qt-project.org/browse/PYSIDE-129
>
> This bug is only half of the story:
>
> We had that simple @staticmethod work-around.
> But actually, the main reason seems to be that PySide suffers
> any reference cycle.
>
> The tiny example again breaks as soon as I add a property:
>
> {code}
>      from PySide.QtCore import QAbstractTableModel, QObject
>
>      class MyModel(QAbstractTableModel):
>          def __init__(self, *args):
>              super(MyModel, self).__init__(*args)
>              self.destroyed.connect(self.onDestroy)
>
>          @staticmethod
>          def onDestroy():
>              print('destroyed')
>
>          @property
>          def hugo(self):
>              return 42
>
>      m = MyModel()
>      del m
> {code}
>
> Remove the @property, and it works, again.
>
> Rule of thumb:
> If you need anything like a property, use a normal Python class
> and put your Qt object into it as an attribute. Otherwise your
> program will tend to grow in memory ;-)
>
> Stefan, I added this code to your bug report.
>
> cheers - chris
>
> --
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