[PyQt] QML, pyqtSlot and ownership

Nenad Ognjanovic bgr.gyk at gmail.com
Mon Jul 14 12:59:42 BST 2014

I'd be very grateful if someone could confirm whether the solution to the
following two scenarios is valid:

I'd like to be able to call a pyqtSlot from QML, have the slot instantiate
and return a QObject, and have the QML take over the ownership of the
object, without having to keep an explicit reference to the object on the
python side. Without explicitly using sip.transferto (as suggested in this
stackoverflow answer http://stackoverflow.com/a/24696295/858766 - the
question was posted by me, please go through it if you have time), the
object is garbage collected before it even reaches the QML side.
Is using sip.transferto the correct approach here?

Additionally, there's a similar problem when the situation is reversed. If
I expose a class Dummy via qmlRegisterType and then instantiate it in QML
using Dummy {} (e.g. as a ListView delegate), a segfault will occur as soon
as the delegate gets out of ListView visible area and gets destroyed. In
this case I can add sip.transferto in the constructor but it doesn't seem
right, and it also causes the process to not quit after the event loop
exits. Should I transfer the ownership back?
Is sip.transferto even something a PyQt user should be using and be aware

In both cases I don't see the destructors being triggered (by the fact that
there's no print output from the overridden __del__ and connected destroyed
signal handler), which contributes to my concern about the approach.
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