PyQt5 MainWindow with flag instantly goes out of scope

Question

I created UI using Qt Designer. Then I converted the ui file to a .py file (pyuic -x) - it works fine if launched directly. Then I tried to subclass my ui in a separate file to implement additional logic. And this is where things start to go wrong. Inheriting from QMainWindow and my Qt Designer file works OK with no issues, as expected. However, the moment I set any WindowFlag for my QMainWindow (any flag - I tried these: StaysOnTop, FramelessWindowHint) and run the file, the window appears and instantly disappears. The program continues to run in a console window, or as a PyCharm process, but the window is gone. It looks to me like it is getting out of scope - but why setting a simple flag would make any difference to the garbage collector? Could someone explain this behaviour?

Minimum code required to reproduce this phenomenon:

from ui import Ui_MainWindow
from PyQt5 import QtCore, QtWidgets, QtGui
import sys


class Logic(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self):
        QtWidgets.QMainWindow.__init__(self)
        self.setupUi(self)
        self.show()
        # self.setWindowFlags(QtCore.Qt.WindowStaysOnTopHint)
        # self.setWindowFlags(QtCore.Qt.FramelessWindowHint)
        self.setAttribute(QtCore.Qt.WA_NoSystemBackground, True)
        self.setAttribute(QtCore.Qt.WA_TranslucentBackground, True)
        

if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    window = Logic()
    sys.exit(app.exec_())

The window should appear and stay on the screen until one (or more) of the flags are uncommented. I use Python 3.8 (32-bit) with PyQt5. Run environment provided by PyCharm. Windows 10.

Answer 1

From the documentation of setWindowFlags():

Note: This function calls setParent() when changing the flags for a window, causing the widget to be hidden. You must call show() to make the widget visible again..

So, just move self.show() after setting the flags, or call it from outside the __init__ (after the instance is created), which is the most common and suggested way to do so, as it's considered good practice to show a widget only after it has been instanciated.