[PyQt] Weird behaviour of destroyed() slots

[Phil Thompson]

On Sat, 26 Jul 2008 21:07:09 +0200, Giovanni Bajo <rasky at develer.com>
wrote:
> On Sat, 2008-07-26 at 17:10 +0100, Phil Thompson wrote:
>> On Wed, 23 Jul 2008 15:42:18 +0200, Giovanni Bajo <rasky at develer.com>
>> wrote:
>> > Hi Phil:
>> > 
>> >
>>
>
=============================================================================
>> > import sip
>> > from PyQt4.Qt import *
>> > 
>> > called = []
>> > 
>> > class Core(QObject):
>> >      def __init__(self, parent=None):
>> >          QWidget.__init__(self, parent)
>> >          QObject.connect(self, SIGNAL("destroyed()"), self.callback)
>> >          QObject.connect(self, SIGNAL("destroyed()"), lambda: 
>> > self.callback())
>> >      def callback(self):
>> >          called.append("done")
>> > 
>> > app = QApplication([])
>> > core = Core(app)
>> > sip.delete(core)
>> > 
>> > assert len(called) == 2, called
>> >
>>
>
=============================================================================
>> > Traceback (most recent call last):
>> >    File "bugpyqt.py", line 18, in <module>
>> >      assert len(called) == 2, called
>> > AssertionError: ['done']
>> > 
>> > The slot with "lambda" is called, but the other one is not.
>> 
>> The reason is that PyQt knows that self is being destroyed and
>> self.callback may be a wrapped C++ method (although it isn't in this
> case)
>> so it won't invoke it.
> 
> Why does it refuse to invoke a wrapped C++ method? Even if self is being
> destroyed (that is: within its C++ destructor) is still valid to invoke
> methods AFAICT. After all, it's something that's perfectly doable in C++
> as well. Or not?

I had second thoughts after my first response. A wrapped C++ method will
check to see if the C++ instance has been destroyed anyway, so there is no
need to check it here as well.

However, a wrapped C++ method still won't work in exactly the same way as a
C++ application because the "C++ instance has been destroyed" flag has to
be set before the destroyed() signal has been emitted. But it's ok for pure
Python methods.

Phil