class MainWindow(QtGui.QMainWindow):

    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)
QtCore.QTimer.singleShot(5000, self.showChildWindow)

def showChildWindow(self):
    self.child_win = ChildWindow(self)
self.child_win.show()
if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
myapp = MainWindow()
myapp.show()
sys.exit(app.exec_())
Open a GUI file from another file PyQT