# 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: {}
}