MainWindow.py(this is the window that contains the menu)

from PyQt4 import uic

(Ui_MainWindow, QMainWindow) = uic.loadUiType('mainwindow.ui')

class MainWindow (QMainWindow):
    """MainWindow inherits QMainWindow"""

    def __init__ (self, parent = None):
        QMainWindow.__init__(self, parent)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)

    def __del__ (self):
        self.ui = None