[PyQt] Ownership disconnect with "temporary" python objects?

[Johan Fläckman]

Thanks for the quick reply.
I understand that I can work around it by making sure the instance does not
die within the addMenu-scope.
I kind of would have expected the QAction to pick up on the fact that its
C++ counterpart is destroyed though (and not crash).

Any idea why this is not a problem with python3?


On Thu, Apr 16, 2015 at 9:49 AM, Phil Thompson <phil at riverbankcomputing.com>
wrote:

> On 16/04/2015 8:32 am, Johan Fläckman wrote:
>
>> Hi, I have a strange crash in PyQt5 and I'm not sure if the error is in my
>> buildscripts or in PyQt.
>> I have built Qt5, PyQt5 and SIP myself with msvc2010 / python 2.7
>>
>> PyQt 5.4.1
>> Qt 5.4.1
>> SIP 4.16.7
>>
>> My compiled version seems to work fine in almost all cases but I have this
>> one crash that shows something went wrong.
>> Here's a minimal script that reproduce the crash (imports etc omitted):
>>
>> *a = PyQt5.QtWidgets.QApplication(sys.argv)*
>> *w = PyQt5.QtWidgets.QMainWindow();*
>>
>> *res = w.menuBar().addMenu( PyQt5.QtWidgets.QMenu( "foo" ) )*
>>
>> *print **res**                     # <---- PyQt5.QtWidgets.QAction object
>> at 0x000000000279E3A8*
>> *sip.isdestroyed(res)    # <---- False*
>> *sip.ispyowned(res)     # <----- False*
>> *sip.dump(res)            # <----- Crash (res already destroyed in C++)*
>>
>>
>> From what I can tell from debugging:
>> The QMenu instance passed to addMenu is deconstructed (both in python and
>> in C++) before res is assigned in python.
>> The menu's QAction is destroyed in C++ during (or right after) the
>> deconstruction of the QMenu but res is assigned a python instance of a
>> QAction that thinks this C++ representation still exists.
>>
>> I can *not *reproduce this crash with the prebuilt python 3.4 and the
>> riverbank-pyqt5 binaries but I *can *reproduce it with any prebuilt
>> "unofficial" pyqt5/python27 binaries I could find scattered around the
>> internet. The crash is reproducible in both debug and non-debug builds.
>>
>> Has anyone else encountered this or can confirm that their python27/pyqt5
>> version can run this code?
>>
>
> It's a bug in your code. QMenuBar.addMenu(QMenu) does not take ownership
> of the menu. Give the menu a parent or keep an explicit reference to it.
>
> Phil
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