[PyQt] subtle bug in PyQt in combination with Python garbage collector

[Phil Thompson]

On Sat, 13 Aug 2011 10:05:14 -0600, Kovid Goyal <kovid at kovidgoyal.net>
wrote:
> This bug has been present for a very long time. As a workaround in my
> projects, 
> I disable the automatic garbage collector and run garbage collection
> manually
> in the GUI thread via QTimer. Here's the code to do that:
> 
> class GarbageCollector(QObject):
> 
>     '''
>     Disable automatic garbage collection and instead collect manually
>     every INTERVAL milliseconds.
> 
>     This is done to ensure that garbage collection only happens in the
GUI
>     thread, as otherwise Qt can crash.
>     '''
> 
>     INTERVAL = 5000
> 
>     def __init__(self, parent, debug=False):
>         QObject.__init__(self, parent)
>         self.debug = debug
> 
>         self.timer = QTimer(self)
>         self.timer.timeout.connect(self.check)
> 
>         self.threshold = gc.get_threshold()
>         gc.disable()
>         self.timer.start(self.INTERVAL)
>         #gc.set_debug(gc.DEBUG_SAVEALL)
> 
>     def check(self):
>         #return self.debug_cycles()
>         l0, l1, l2 = gc.get_count()
>         if self.debug:
>             print ('gc_check called:', l0, l1, l2)
>         if l0 > self.threshold[0]:
>             num = gc.collect(0)
>             if self.debug:
>                 print ('collecting gen 0, found:', num, 'unreachable')
>             if l1 > self.threshold[1]:
>                 num = gc.collect(1)
>                 if self.debug:
>                     print ('collecting gen 1, found:', num,
'unreachable')
>                 if l2 > self.threshold[2]:
>                     num = gc.collect(2)
>                     if self.debug:
>                         print ('collecting gen 2, found:', num,
>                         'unreachable')
> 
>     def debug_cycles(self):
>         gc.collect()
>         for obj in gc.garbage:
>             print (obj, repr(obj), type(obj))
> 
> Kovid.

Thanks for the insight.

Two solutions spring to mind...

1. Implement the above so that it is created when QThread.start() is
called for the first time.

2. Change the SIP generated dtor code so that it calls deleteLater() if
the object's thread is not the current one...

    if (QThread::currentThread() == obj->thread())
        delete obj;
    else
        obj->deleteLater();

I'd prefer the latter (assuming it works).

Thoughts?

Phil