PySide2 QUiLoader returns an empty window

Question

PySide2(5.6.0~a1) Qt UI file loader returns an empty window whereare PyQt5 loader works fine. Could you explained to me where I am wrong.

Non Working PySide2 version:

import sys
from PySide2.QtWidgets import QDialog, QApplication
from PySide2 import QtUiTools

class AppWindow(QDialog):
    def __init__(self):
        super().__init__()
        self.ui = QtUiTools.QUiLoader().load("dialog1.ui")
        self.show()

app = QApplication(sys.argv)
w = AppWindow()
sys.exit(app.exec_())

Working PyQt5 version:

import sys
from PyQt5.QtWidgets import QDialog, QApplication
from PyQt5 import uic

class AppWindow(QDialog):
    def __init__(self):
        super().__init__()
        self.ui = uic.loadUi("dialog1.ui", self)
        self.show()

app = QApplication(sys.argv)
w = AppWindow()
sys.exit(app.exec_())

Using this function also does not work :

def loadUiWidget(uifilename, parent=None):
    loader = QtUiTools.QUiLoader()
    uifile = QtCore.QFile(uifilename)
    uifile.open(QtCore.QFile.ReadOnly)
    ui = loader.load(uifile, parent)
    uifile.close()
    return ui

Answer 1

In PySide2 there's no function to QMainWindow class overwrite itself. It's necessary to show the ui:

import sys
from PySide2.QtWidgets import QDialog, QApplication
from PySide2 import QtUiTools

class AppWindow(QDialog):
    def __init__(self):
        super().__init__()
        self.ui = QtUiTools.QUiLoader().load("dialog1.ui")
        self.ui.show()

app = QApplication(sys.argv)
w = AppWindow()
sys.exit(app.exec_())

Answer 2

If you want to load QMainWindow from designer *.ui file you can use

import sys
from PySide2.QtWidgets import QApplication, QMainWindow
from PySide2.QtUiTools import QUiLoader

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setCentralWidget(QUiLoader().load("form.ui"))

but slot-signal bindings, which are set in the designer in *.ui file, are not working anyway.

So, for full-function use of designer GUI and slot-signal bindings, the only way I found is to compile *.ui file to python module with pyside UI compiler:

pyside2-uic mainwindow.ui > ui_mainwindow.py

and then include produced ui_mainwindow. In this method the slot-signal pairs from Qt UI designer will work well.

import sys
from PySide2.QtWidgets import QApplication, QMainWindow
from ui_mainwindow import Ui_MainWindow

class MainWindow(QMainWindow, Ui_MainWindow):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.setupUi(self)