class StartQT4(QtGui.QMainWindow): def __init__(self, parent=None): QtGui.QWidget.__init__(self, parent) self.ui = Ui_MainWindow() self.ui.setupUi(self) QtCore.QObject.connect(self.ui.pushButton, QtCore.SIGNAL("clicked()"), self.but_clked ) def but_clked(self): self.ui.lineEdit_3.setText(str(int(self.ui.lineEdit.text())+int(self.ui.lineEdit_2.text())))