python3 PyQt4:如何切换QStackedWidgets，使用QTDesigner构建

import sys
from PyQt4 import QtGui, QtCore

class MainWindow(QtGui.QMainWindow):
    def __init__(self):
        super(MainWindow, self).__init__()

        self.stacked_widget = QtGui.QStackedWidget()
        self.setCentralWidget(self.stacked_widget)

        self.page1 = QtGui.QWidget()
        self.page2 = QtGui.QWidget()

        self.stacked_widget.addWidget(self.page1)
        self.stacked_widget.addWidget(self.page2)

        self.button1 = QtGui.QPushButton("Page 1", self.page1)
        self.button1.clicked.connect(self.show_page1)

        self.button2 = QtGui.QPushButton("Page 2", self.page2)
        self.button2.clicked.connect(self.show_page2)

    def show_page1(self):
        self.stacked_widget.setCurrentIndex(0)

    def show_page2(self):
        self.stacked_widget.setCurrentIndex(1)

if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    window = MainWindow()
    window.show()
    sys.exit(app.exec_())