Рыба (эталон) главного скрипта при подключении внешнего UI, сгенеренного QtDesigner'ом

pyqt5_stub_with_ui_gen_scr.MD

The Standard way in main script with external UI

(previously generated by QT Designer / Creator)

NEW: MDI (Multi-dialog interface) is supported [inside 2nd comment]

# -*- coding: utf-8 -*-

import sys
from PyQt5.QtWidgets import QApplication, QWidget
from my_app_ui import Ui_Form


class MyWindow(QWidget, Ui_Form):  # Inheritance exactly in this sequence
    def __init__(self, *args, **kwargs):
        super(MyWindow, self).__init__(*args, **kwargs)

        self.setupUi(self)

        # -== Signals ==-
        self.btn_click.clicked.connect(self.click_me)

        self.show()

    # -== event handlers aka Slots ==-
    def click_me(self):
        my_text = self.lineEdit.text()
        self.label.setText(my_text)


if __name__ == "__main__":
    app = QApplication(sys.argv)  # or QApplication([])
    win = MyWindow()
    sys.exit(app.exec())  # or simply app.exec() or app.exec_()

Awesome way to create MDI (multi-dialog interface)
(supported MDI app, QMainWindow-based)