如何使用PyQt在窗口中切换布局？（没有关闭打开窗口）

from PyQt4 import QtCore, QtGuiclass MainWindow(QtGui.QMainWindow):    def __init__(self, parent=None):        super(MainWindow, self).__init__(parent)        self.central_widget = QtGui.QStackedWidget()        self.setCentralWidget(self.central_widget)        login_widget = LoginWidget(self)        login_widget.button.clicked.connect(self.login)        self.central_widget.addWidget(login_widget)    def login(self):        logged_in_widget = LoggedWidget(self)        self.central_widget.addWidget(logged_in_widget)        self.central_widget.setCurrentWidget(logged_in_widget)class LoginWidget(QtGui.QWidget):    def __init__(self, parent=None):        super(LoginWidget, self).__init__(parent)        layout = QtGui.QHBoxLayout()        self.button = QtGui.QPushButton('Login')        layout.addWidget(self.button)        self.setLayout(layout)        # you might want to do self.button.click.connect(self.parent().login) hereclass LoggedWidget(QtGui.QWidget):    def __init__(self, parent=None):        super(LoggedWidget, self).__init__(parent)        layout = QtGui.QHBoxLayout()        self.label = QtGui.QLabel('logged in!')        layout.addWidget(self.label)        self.setLayout(layout)if __name__ == '__main__':    app = QtGui.QApplication([])    window = MainWindow()    window.show()    app.exec_()