PySide / PyQt detect if user trying to close window

Override the closeEvent method of QWidget in your main window.

For example:

class MainWindow(QWidget): # or QMainWindow
    ...

    def closeEvent(self, event):
        # do stuff
        if can_exit:
            event.accept() # let the window close
        else:
            event.ignore()

Another possibility is to use the QApplication‘s aboutToQuit signal like this:

app = QApplication(sys.argv)
app.aboutToQuit.connect(myExitHandler) # myExitHandler is a callable