[PySide] QObject.destroyed() is not emitted

[Alexey Vihorev]

Yes, I tried something like that:

def onDestroy(*args):
    print('destroyed')

obj = QtCore.QObject()
obj.destroyed.connect(onDestroy)

obj2 = QtCore.QObject(obj)
obj2.destroyed.connect(onDestroy)

del(obj)

In this case onDestroy() is called twice, just as it should. But once again
- omit the last line and nothing Is called. So for it to work explicit
destruction must be called at the top of the chain. Which is problematic in
my case.

-----Original Message-----
From: Christian Tismer [mailto:tismer at stackless.com] 
Sent: 29 ноября 2012 г. 0:58
To: Alexey Vihorev
Cc: 'Stephan Deibel'; pyside at qt-project.org
Subject: Re: [PySide] QObject.destroyed() is not emitted

Hi Alexey,

I tried your example and added a parent.
Right now it looks exactly as it should be:

 >>> def onDestroy(*args):
...     print('destroyed')
...
 >>> obj = QtCore.QObject()
 >>> obj.destroyed.connect(onDestroy)
True
 >>> par = QtCore.QObject()
 >>> obj.setParent(par)
 >>> del obj
 >>> del par
destroyed
 >>>

This is PySide on Mac OS X,
 >>> PySide.__version__
'1.1.2'
 >>> QtCore.__version__
'4.8.3'
 >>>

Maybe you have some more references to the object?
For simple refcount checking, you can use sys.getrefcount .

 >>> g=sys.getrefcount
 >>> g(obj)
2

Note that this prints 2 because of the reference from the call. So the
object should go away when the one real reference goes away.
Maybe you have a different problem, involving more objects?

---------

By the way, PySide seems to be not happy with reference cycles, while pure
python is ok with that:

 >>> obj = QtCore.QObject()
 >>> obj.setParent(obj)
 >>> g(obj)
2

Eeek, that should be 3.
And if fact.......

 >>> del(obj)
Segmentation fault: 11

ciao -- Chris


On 11/28/12 9:24 PM, Alexey Vihorev wrote:
> Hi!
> Well, I understand that if what I need to do is achievable with 
> explicit deletion,  I should go for it :) The problem is that in my 
> case the deletion of the object is triggered by deletion of another 
> object - its parent. But I can't catch the deletion event - neither of the
object nor its parent.
> That's the problem. The whole reason I started looking into that is 
> that one of my objects needed to free some external resources when
destroyed.
>
> -----Original Message-----
> From: Stephan Deibel [mailto:sdeibel at wingware.com]
> Sent: 28 ноября 2012 г. 16:33
> To: Alexey Vihorev
> Cc: pyside at qt-project.org
> Subject: Re: [PySide] QObject.destroyed() is not emitted
>
> Alexey Vihorev wrote:
>> Got problems with the following code:
>>
>> from PySide import QtCore
>>
>> def onDestroy(*args):
>>
>> print('destroyed')
>>
>> obj = QtCore.QObject()
>>
>> obj.destroyed.connect(onDestroy)
>>
>> The onDestroy() method is not called, unless del(obj) is called 
>> explicitly. In PyQt4 it is called without del(obj). Is that a bug or 
>> intended behavior? I'm using Qt 4.8.2, PySide 1.1.2 32bit, Windows
>> 7-64
>>
> Since no one responded I'll take a stab at this:
>
> I suspect the life cycle of the QObject under PySide is a little 
> different and the instance will be deleted later during garbage 
> collection and not immediately when it goes out of scope. I'm actually 
> not sure why that is happening in PyQt (seems slightly surprising).
>
> In general if you want to make sure an instance is deleted at a 
> particular moment in time, you need to delete it explicitly. By that, 
> I mean calling some sort of delete method (in PySide I think it's
> shiboken.delete(obj) and not just "del obj". The latter just deletes 
> your reference to the instance but doesn't destroy the instance until 
> all other references are gone and it is garbage collected. Of course 
> calling
> shiboken.delete(obj) will be a problem if you still have other 
> references and try to use them!
>
> There is also obj.deleteLater() which marks an object for deletion but 
> it's not deleted until the event loop is reached again. Depending on 
> what you're doing this may be a safer way to delete it.
>
> At first I thought this might be a refcount bug in PySide. However, 
> I'm thinking not since "del obj" just deletes your reference to it and 
> if there were extra references hanging around (either on purpose or as 
> a result of a refcount bug) then it would not be deleting the instance 
> at all even after "del obj".
>
> I don't understand PySide internals that well so it's possible I'm 
> missing some subtlety here. I'm going more on my knowledge of Python in
general.
>
> - Stephan
>
> _______________________________________________
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> PySide at qt-project.org
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