[PyKDE] Widget Counting and Garbage Collecting

[Phil Thompson]

On Friday 21 January 2005 7:42 pm, bdw wrote:
> I have always run my PyQt apps with Qt's -widgetcount flag because it
> seemed to be good at picking up things like circular references in my
> code.
>
> While using QScintilla recently, though, I started seeing some
> inconsistency in Qt's reports - sometimes it would show one widget left
> undestroyed, sometimes not. I eventually tracked this down to
> QScintilla's built-in context menu. Qt only reports one widget left if
> the context menu is shown.
>
> Here is a minimal QScintilla app which also uses Python's garbage
> collecting module:
>
>   import sys, gc
>   from qt import *
>   from qtext import QextScintilla
>   if __name__ == "__main__":
>       gc.set_debug(gc.DEBUG_LEAK|gc.DEBUG_STATS)
>       a = QApplication(sys.argv)
>       a.connect(a,SIGNAL('lastWindowClosed()'),a,SLOT('quit()'))
>       q = QextScintilla(); a.setMainWidget(q); q.show()
>       a.exec_loop()
>       del q
>       gc.collect(); del gc.garbage[:]
>
> If I run this, right-click the editor, then close it, I get the
> following output:
>
>   [bdw at localhost test]$ python qs-min.py -widgetcount
>   gc: collecting generation 2...
>   gc: objects in each generation: 492 2762 0
>   gc: done.
>   gc: collecting generation 2...
>   gc: objects in each generation: 0 0 3248
>   gc: done.
>   Widgets left: 1    Max widgets: 6
>
> This seems to show that Python collects all its garbage, but Qt does
> not destroy all its Widgets. Or is it that Python killed the stray
> Widget and Qt is reporting out-of-date information?

QScintilla wasn't deleting the popup when it itself was being destroyed - 
fixed in the next snapshot.

> Can I rely on the -widgetcount flag when debugging my PyQt apps, or am
> I going about this in completely the wrong way?

From the above it looks like the count is reliable.

> As a final test, I tried running eric with the -widgetcount flag and
> got the following output:
>
>   [bdw at localhost eric-3.5.1]$ python eric3/eric3.py -widgetcount
>   Widgets left: 650    Max widgets: 654
>
> Now that can't be right - or can it!?

I think there is a difference between a widget and an application. An 
individual widget shouldn't leak memory (as QScintilla was), but I wouldn't 
expect an application to tidy up on exit in the same way. An application that 
leaks memory during use is a different matter - but that's not what your 
figures measure.

Phil