How to prevent PyQt objects from garbage collecting from different thread?

Question

I am using Qt 4.7 and PyQt 4.7 to build a multi-threaded GUI program. I've been carefully managing PyQt objects so that they stay in one UI thread to avoid synchronization issues and there is no problem in general.

But sometimes, at the moment the python garbage collector is activated from other thread, the destructor of Qt object is called right there and the following assertion fails from inside Qt.

I can define QT_NO_DEBUG even for the debug version and it should be fine because objects being collected hardly cause a synchronization problem. But still, I don't think that's a good idea to turn off other debug messages. How do I prevent this from happening?

#if !defined (QT_NO_DEBUG) || defined (QT_MAC_FRAMEWORK_BUILD)
void QCoreApplicationPrivate::checkReceiverThread(QObject *receiver)
{
    QThread *currentThread = QThread::currentThread();
    QThread *thr = receiver->thread();
    Q_ASSERT_X(currentThread == thr || !thr,
               "QCoreApplication::sendEvent",
               QString::fromLatin1("Cannot send events to objects owned by a different thread. "
                                   "Current thread %1. Receiver '%2' (of type '%3') was created in thread %4")
               .arg(QString::number((quintptr) currentThread, 16))
               .arg(receiver->objectName())
               .arg(QLatin1String(receiver->metaObject()->className()))
               .arg(QString::number((quintptr) thr, 16))
               .toLocal8Bit().data());
    Q_UNUSED(currentThread);
    Q_UNUSED(thr);
}
#elif defined(Q_OS_SYMBIAN) && defined (QT_NO_DEBUG)

Adrian Buehlmann · Accepted Answer · 2011-08-15 13:10:28Z

6

This problem popped up on the PyQt mailing list in the thread

"subtle bug in PyQt in combination with Python garbage collector"
http://www.riverbankcomputing.com/pipermail/pyqt/2011-August/030376.html

Kovid Goyal proposed a workaround in
http://www.riverbankcomputing.com/pipermail/pyqt/2011-August/030378.html

where he wrote:

As a workaround in my projects, I disable the automatic garbage collector and run garbage collection manually in the GUI thread via QTimer.

He posted a code example there.

answered Aug 15, 2011 at 13:10

Adrian Buehlmann

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Comments

Prof.Ebral · Accepted Answer · 2010-11-09 05:43:28Z

1

I would recommend using pyqtSignal. You can create signals for your threads to send when their task is completed and the receiver becomes the signal's connected function.

answered Nov 9, 2010 at 5:43

Prof.Ebral

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