If you are a Python developer on a Mac, here is a nasty surprise: The latest PyQt5 release (5.15) doesn’t work on Big Sur (yet).
When you start your Python application, all you see is a blank slate and your CPU running at 100%.
The workaround is simple: Downgrade your PyQt5 to 5.13.
pip install PyQt5==5.13
This solved the issue on three of my Python applications: