PySide6 with Python C API: EXC_BAD_ACCESS error when calling a method with inherited class from QRasterWindow

Blady

Environment:
Python version: 3.9.6 (default, May 7 2023, 23:32:45)
PySide6 version: 6.7.0

When I create an instance of QRasterWindow, calling the metric method is ok:

Py_Initialize();

    PyObject * GUI = PyImport_ImportModule("PySide6.QtGui");
    if(!GUI) {
        printf("\nCould not import PySide6.QtGui module.\n");
        return 0;
    }

    // QApplication
    PyObject* QGA = PyObject_GetAttrString(GUI, "QGuiApplication");
    PyObject * argsqa = PyTuple_New (1);
    PyObject * listqa = PyList_New (1);
    PyList_SetItem (listqa, 0, PyUnicode_FromString (""));
    PyTuple_SetItem (argsqa, 0, listqa);
    PyObject * res = PyObject_CallObject(QGA, argsqa);

    // QRasterWindow
    PyObject* QRW = PyObject_GetAttrString(GUI, "QRasterWindow");
    if(QRW) {

        // Create instance
        PyObject * args = PyTuple_New (0);
        PyObject * QRW_inst = PyObject_CallObject(QRW, args);
             
        // Call metric
        PyObject * mm = PyObject_GetAttrString(QRW_inst, "metric");

        PyObject * uu = PyObject_GetAttrString(GUI, "QPaintDevice");
        PyObject * ww = PyObject_GetAttrString(uu, "PdmWidth");

        PyObject * argsm = PyTuple_New (1);
        PyTuple_SetItem (argsm, 0, ww);
        PyObject * res = PyObject_CallObject(mm, argsm);

        int val = PyLong_AsLong(res);
        printf("\nQRasterWindow.metric value is %d \n", val);

    } else {
        printf("\nCould not get QRW class.\n");
    }

But if I create a class PCJ inherited from QRasterWindow then calling metric method gives segmentation fault EXC_BAD_ACCESS:

// Create PCJ class inherited from QRasterWindow
    PyType_Slot slots[] = {
        { Py_tp_doc, "PGJ" },
        { Py_tp_base, QRW },
        { Py_tp_init, QRW_init },
        { 0 },
    };
    PyType_Spec spec = { "PCJ", 0, 0,
                          Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, slots };
    PyTypeObject *PCJ = (PyTypeObject *)PyType_FromSpec(&spec);
    printf("\nPCJ value is %d \n", PCJ);

    // Create PCJ instance
    PyObject * argsj = PyTuple_New (0);
    PyObject * PCJ_inst = PyObject_CallObject((PyObject *) PCJ, argsj);
    printf("\nPCJ_inst value is %d \n", PCJ_inst);
         
    // Call metric
    PyObject * mm = PyObject_GetAttrString(PCJ_inst, "metric");
    printf("\nmm value is %d \n", mm);

    PyObject * uu = PyObject_GetAttrString(GUI, "QPaintDevice");
    printf("\nuu value is %d \n", uu);
    PyObject * ww = PyObject_GetAttrString(uu, "PdmWidth");
    printf("\nww value is %d \n", ww);

    PyObject * argsm = PyTuple_New (1);
    PyTuple_SetItem (argsm, 0, ww);
    PyObject * resj = PyObject_CallObject(mm, argsm); // segmentation fault
    printf("\nresj value is %d \n", resj);

    int valj = PyLong_AsLong(resj);
    printf("\nPCJ.metric value is %d \n", valj);

Output:

QRasterWindow.metric value is 0 
PCJ value is 436267760 
In QRW_init 
PCJ_inst value is 263891520 
mm value is 263896816 
uu value is 439183584 
ww value is 263877728 
zsh: segmentation fault

What is wrong?

SGaist

Hi and welcome to devnet,

Excuse my silly question but why go through the length of using PySide6 using Python C API ? Why not write your application directly in C++ ?

Blady

Hi and thanks for your welcome.
Good question, I'm trying to create Qt bindings for Ada language.
Some attempts with C++ were failing due to some C++ subtleties.
But Python provides C API which I can used easily with Ada.
I provide some very first Qt bindings for Ada.
But class inheritance is still in Python.
I wanted to make them full Ada. I'm able to instantiate a class inherited from QRasterWindow (for instance) but a call to the method metric (for instance) provoques a seg fault.
(for simplicity I post the code in C)