In PySide6, why assignment before super().__init__() might lead to abnormal result?

[isaacchangwang]

Class A is derived from QObject, and class B is derived from A. In the GUI, when the OK button is clicked, the method B.log() is invoked, and it prints 'Hi' in the console. Here is the code:

main.py

import sys
from PySide6.QtCore import QObject, QTimer, QUrl, Slot
from PySide6.QtGui import QGuiApplication
from PySide6.QtQml import QmlElement, QQmlApplicationEngine

QML_IMPORT_NAME = 'PyLib'
QML_IMPORT_MAJOR_VERSION = 1


class A(QObject):
    def __init__(self):
        super().__init__()


@QmlElement
class B(A):
    def __init__(self):
        super().__init__()

    @Slot()
    def log(self):
        print('Hi')


def main():
    app = QGuiApplication(sys.argv)
    engine = QQmlApplicationEngine()
    engine.load(QUrl('main.qml'))
    if not engine.rootObjects():
        sys.exit(-1)
    sys.exit(app.exec())


if __name__ == '__main__':
    main()

main.qml

import QtQuick
import QtQuick.Controls
import PyLib

ApplicationWindow {
    height: 100
    width: 100
    visible: true

    Button {
        text: "OK"
        onClicked: b.log()
    }

    B {
        id: b
    }
}

Then I find something I cannot explain:

1. If I set an instance variable to a QTimer object before super().init() in the init() method of class B, i.e.,

@QmlElement
class B(A):
    def __init__(self):
        self.t = QTimer()
        super().__init__()

    @Slot()
    def log(self):
        print('Hi')

when the OK button is clicked, no 'Hi' is printed. However, if I set the instance variable to an int or a string, e.g.,

@QmlElement
class B(A):
    def __init__(self):
        self.t = 1
        super().__init__()

    @Slot()
    def log(self):
        print('Hi')

when the OK button is clicked, 'Hi' is printed.

2. If a QTimer object is assigned to a local variable instead of an instance variable, i.e.,

@QmlElement
class B(A):
    def __init__(self):
        t = QTimer()
        super().__init__()

    @Slot()
    def log(self):
        print('Hi')

the GUI window won't be shown.

It seems super().init() must be called first in the initialization method, otherwise it might lead to abnormal result. Is that true?

[JonB]

@isaacchangwang said in In PySide6, why assignment before super().__init__() might lead to abnormal result?:

It seems super().init() must be called first in the initialization method, otherwise it might lead to abnormal result. Is that true?

I would always expect to have to call the base super().__init__() as the first statement in any derived class, whether Qt/QObject or not. Putting a QObject/QTimer into a derived class as a member before it has been initialised does not sound like a good idea.