PyQt5 Calculator Gui Tutorial Part 4 

Make sure you have the `equals_pressed()` slot defined correctly and that the equal button is connected to that slot. It is likely that the code in `equals_pressed()` has an error and the program crashes when that code is run.

Are you using PyQt5? And did you call `setCheckable(True)` for the pushButtons in the __init__ function?

Watch from 3:10, I think you're missing the `setCheckable(True)` part.

Thanks, it works