问PyQt MainWindow没有显示小部件

EN

import sys
from PyQt4 import QtGui, QtCore

class MainWindow(QtGui.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent=parent)
        self.resize(300, 400)
        self.centralWidget = QtGui.QWidget(self)
        self.hbox = QtGui.QHBoxLayout(self.centralWidget)
        self.setLayout(self.hbox)

        names = ['button1', 'button2', 'button3']
        testButtons = buttonFactory(names, parent=self)
        self.hbox.addWidget(testButtons)

class buttonFactory(QtGui.QWidget):
    def __init__(self, names, parent=None):
        super(buttonFactory, self).__init__(parent=parent)
        self.vbox = QtGui.QVBoxLayout()
        self.setLayout(self.vbox)
        for name in names:
            btn = QtGui.QPushButton(name)
            self.vbox.addWidget(btn)

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    gui = MainWindow()
    gui.show()
    app.exec_()