qmlRegisterTypeSingleton with contextData error while using PySide6

RLayman

I've been having this issue for a long time now. The existing code worked fine for years and didn't start failing until after a QT update over six months ago. If you register a singleton using PySide6, you will get a 'Cannot assign object to list property "contextData"' error message when the QML code has a ListModel. This happens even if the singleton is not being used by the QML code. If you remove qmlRegisterSingletonType, the error goes away. I've searched the internet and have not seen this error happening due to adding a singleton, so I'm not sure what's going on.

# singleton.py

from PySide6.QtCore import Qt
from PySide6.QtCore import QObject

class Singleton(QObject):

    def __init__(self):
        super().__init__()

# app

import sys
from PySide6.QtCore import Qt, QObject, QAbstractListModel, QUrl, QByteArray, Signal, Slot, Property
from PySide6.QtGui import QGuiApplication
from PySide6.QtQml import QQmlApplicationEngine
from PySide6 import QtQml
from singleton import Singleton 

_singleton = Singleton()

def singleton_callback(self):
    return _singleton

if __name__ == "__main__":
    app = QGuiApplication(sys.argv)
    engine = QQmlApplicationEngine()

    # This Fails 
    QtQml.qmlRegisterSingletonType(Singleton, 'Test', 1, 0, 'Singleton',singleton_callback)

    # This Works
    #QtQml.qmlRegisterType(Singleton, 'Test', 1, 0, 'Singleton')

    engine.quit.connect(app.quit)
    engine.load('main.qml')
    sys.exit(app.exec())

// main.qml
import QtQuick
import QtQuick.Controls
import Test 1.0

ApplicationWindow {
    id: main

    width: 450 
    height: 450 
    title: "Singleton Test" 
    visible: true

    ListModel { id: myModel }

    Component.onCompleted: {}
}

SGaist

You are doing things out of order.

You need to register the Singleton before creating the engine.

SGaist

Hi,

Do you have the same issue with PySide6 ?

RLayman

I'm assuming this is an issue between PySide6 and Qt, but I'm not aware of who is to blame.

SGaist

Sorry, I misread something and thought you were using PyQt6...

Which version of PySide6 are you using ?
How did you install it ?

RLayman

This post is deleted!

RLayman

I am using Pyside 6.8.1.1-2 with Qt 6.8.1-1 on an ArchLinux system. This issue has been going on for over six months, and it's been updated several times since then, so it's not a recent issue. I've also wiped the whole system and done a clean install, but I still have the same issue.

SGaist

You are doing things out of order.

You need to register the Singleton before creating the engine.

RLayman

Sorry about the late reply; I got busy with work. That fixed it! It's kind of odd since it always worked fine up until a few months ago. I'll have to look in the repo to see if something was changed that I forgot about, but I don't remember changing anything in the source at that time. Anyway, thank you for the help!

SGaist

You're welcome !

You know, inverting lines can happen quickly and innocuously :-)
Anyway, now you're back on track.