from customwidget import Ui_customWidget
from mainwindow import Ui_MainWindow

self.ui = Ui_MainWindow()
self.ui.setupUi(self)

self.centralwidget = QtGui.QWidget()
self.customwidget = Ui_customWidget()
self.customwidget.setupUi(self.centralwidget)
self.setCentralWidget(self.centralwidget)