#!/usr/bin/env python
# -*- coding:utf-8 -*-
import sys
# import PyQt4 QtCore and QtGui modules
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4 import uic
(Ui_MainWindow, QMainWindow) = uic.loadUiType('forms/main.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
#-----------------------------------------------------#
if __name__ == '__main__':
    # create application
    app = QApplication(sys.argv)
    app.setApplicationName('SAF')
    # create widget
    w = MainWindow()
    w.setWindowTitle('SAF')
    w.show()
    # connection
    QObject.connect(app, SIGNAL('lastWindowClosed()'), app, SLOT('quit()'))
    # execute application
    sys.exit(app.exec_())

 (addItemForm, QWidget) = uic.loadUiType('forms/addform.ui')
class AddItemsForm(QWidget):
    def __init__(self, parent=None):
        QWidget.__init__(self, parent)
        self.ui = addItemForm()
....
#Другие классы и функции
....
class MainWindow(QMainWindow):
...
...
    def AddForm(self):
        mw = AddItemsForm()
        mw.show()

     def AddForm(self):
        self.mw = AddItemsForm()
        self.mw.show()