pyside2 on macOS Big Sur does not show the main window.

A python GUI project made with pyside2 on macOS Big Sur sometimes does not show the main window. You can see the icon in the tray:

But you see no window and have to force quit the app. The problem can be solved by adding an environment variable.

Solution 1

Put this before creating the main window:

import os
os.environ['QT_MAC_WANTS_LAYER'] = '1'

Full example

import sys
from PySide2.QtWidgets import QApplication
from PySide2.QtWidgets import QLabel

import os
os.environ['QT_MAC_WANTS_LAYER'] = '1'

app = QApplication(sys.argv)
label = QLabel("<center><h1>I must have left my house at eight,<br />because I always do.</h1></center>")
label.show()
app.exec_()

And boom! The window is shown:

Solution 2: Use pyside6

In pyside6 the Layer-backing is always enabled. If you add the code os.environ['QT_MAC_WANTS_LAYER'] = '1' in a pyside6 project, you get the following warning:

Layer-backing is always enabled.  QT_MAC_WANTS_LAYER/_q_mac_wantsLayer has no effect.

Conclusion

Add the line os.environ['QT_MAC_WANTS_LAYER'] = '1' in a pyside2 project or upgrade to pyside6.