class MainWindow(QMainWindow):

    def __init__(self):

        super(MainWindow, self).__init__()

        self.mainWidget = MainWidget(self)
        self.searchWidget = SearchWidget(self)
        self.swidget = QStackedWidget()
        self.swidget.addWidget(self.mainWidget)
        self.swidget.addWidget(self.searchWidget)
        self.setCentralWidget(self.swidget)
        self.initUI()

        ...

class MainWidget(QWidget):

    def __init__(self, parent):

        super(MainWidget,self).__init__(parent)
        self.initUI()

    def initUI(self):

        ...

    def goSearch(self):
        self.parent.swidget.setCurrentWidget(self.parent.searchWidget)

File "/home/maze/Develop/StartApp/startapp.py", line 71, in goSearch
    self.parent.swidget.setCurrentWidget(self.parent.searchWidget)
AttributeError: 'builtin_function_or_method' object has no attribute 'swidget'