empty_dialog.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>EmptyDialog</class>
 <widget class="EmptyDialog" name="EmptyDialog">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>EmptyDialog</string>
  </property>
 </widget>
 <resources/>
 <connections/>
</ui>

testapp.py

import logging
import os

from PySide2.QtWidgets import QApplication, QDialog, QMainWindow, QPushButton, QVBoxLayout, QWidget
from PySide2.QtUiTools import QUiLoader
import PySide2.QtCore


def SomethingWithExternalResources_qt_on_destroyed(obj):
    logging.critical("SomethingWithExternalResources Qt object destroyed")


class SomethingWithExternalResources(PySide2.QtCore.QObject):
    def __init__(self, parent):
        super().__init__(parent)
        logging.critical("External resource __init__ called")
        self.destroyed.connect(SomethingWithExternalResources_qt_on_destroyed)

    def __del__(self):
        logging.critical("External resource __del__ called")


def EmptyDialog_qt_on_destroyed(obj):
    logging.critical("EmptyDialog Qt object destroyed")


class EmptyDialog(QDialog):
    def __init__(self, parent = None):
        super().__init__(parent)
        self.destroyed.connect(EmptyDialog_qt_on_destroyed)
        self.setAttribute(PySide2.QtCore.Qt.WA_DeleteOnClose, True)
        self._resources = SomethingWithExternalResources(self)
        logging.critical("EmptyDialog __init__ called")

    def __del__(self):
        logging.critical("EmptyDialog __del__ called")


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        central_widget = QWidget()
        self.setCentralWidget(central_widget)
        layout = QVBoxLayout(central_widget)

        button_for_code = QPushButton("From code")
        button_for_code.clicked.connect(self._open_code_dialog)
        layout.addWidget(button_for_code)

        button_for_ui_file = QPushButton("From UI file")
        button_for_ui_file.clicked.connect(self._open_ui_dialog)
        layout.addWidget(button_for_ui_file)

    def _open_ui_dialog(self):
        logging.critical("Opening dialog from UI file")
        loader = QUiLoader(self)
        loader.registerCustomWidget(EmptyDialog)

        base_dir = os.path.dirname(os.path.realpath(__file__))
        dialog = loader.load(os.path.join(base_dir, 'empty_dialog.ui'), self)
        dialog.exec_()
        logging.critical("Completed dialog from UI file")

    def _open_code_dialog(self):
        logging.critical("Opening dialog from code")
        dialog = EmptyDialog(self)
        dialog.setWindowTitle("Dialog from code")
        dialog.exec_()
        logging.critical("Completed dialog from code")


def callback(*args, **kwargs):
    logging.critical('MessageHandler: %s %s', args, kwargs)


if __name__ == '__main__':
    logging.critical("PySide2 version: %s", PySide2.__version__)

    # only to suppress a warning about QWebEngine
    PySide2.QtCore.QCoreApplication.setAttribute(PySide2.QtCore.Qt.AA_ShareOpenGLContexts)

    PySide2.QtCore.qInstallMessageHandler(callback)
    app = QApplication([])

    main_window = MainWindow()
    main_window.show()

    app.exec_()