stuff.py

class CustomDialog(QDialog):

    def __init__(self, *args, **kwargs):
        super(CustomDialog, self).__init__(*args, **kwargs)
        
        self.setWindowTitle("HELLO!")
        
        QBtn = QDialogButtonBox.Ok | QDialogButtonBox.Cancel
        
        self.buttonBox = QDialogButtonBox(QBtn)
        self.buttonBox.accepted.connect(self.accept)
        self.buttonBox.rejected.connect(self.reject)

        self.layout = QVBoxLayout()
        self.layout.addWidget(self.buttonBox)
        self.setLayout(self.layout)


class MainWindow(QMainWindow):


    # def __init__ etc.
    # ... not shown for clarity
        
        
    def onMyToolBarButtonClick(self, s):
        print("click", s)
        
        
        dlg = CustomDialog(self)
        if dlg.exec_():  # THIs will kill the app
            print("Success!")
        else:
            print("Cancel!")