Using a .ui file from Qt Designer with Pyside6

main.py

import sys
import os

from PySide6.QtWidgets import QApplication, QMessageBox
from PySide6.QtUiTools import loadUiType
from PySide6.QtGui import QCloseEvent


CWD = os.path.dirname(__file__)

# Use this instead of QUiLoader. PySide6 loader does not allow you to apply
# a UI layout to an existing widget. This prevents you adding custom code for
# the initialization of the widget in a class __init__ block

uiclass, baseclass = loadUiType(os.path.join(CWD, 'mainwindow.ui'))
# baseclass will be the QMainWindow


class MainWindow(uiclass, baseclass):
    def __init__(self):
        super().__init__()
        self.setupUi(self)

    def closeEvent(self, event: QCloseEvent):
        reply = QMessageBox.question(self, 'Leaving...', 'Are you sure you want to quit?',
                                     QMessageBox.Yes | QMessageBox.No, QMessageBox.No)

        if reply == QMessageBox.Yes:
            event.accept()
        else:
            event.ignore()


if __name__ == "__main__":
    app = QApplication(sys.argv)

    window = MainWindow()
    window.show()

    sys.exit(app.exec())