How can I add items in listWidget?

I have trouble adding item in listWidget

I made UI with QT creator and want to import it in python.

When I run this code the items doesn't shows in listWidget, only white blanks.

what is the problem??

please help me

my code is

import sys
from PyQt4 import QtCore, QtGui
from secdialog import Ui_SecDialog

class SecDialog(QtGui.QDialog, Ui_SecDialog):
    def __init__(self, parent=None):
        QDialog.__init__(self, parent)
        self.setupUi(self)
        self.listWidget.addItem("QlistItem_1");
        self.listWidget.addItem("QlistItem_2");
        self.listWidget.addItem("QlistItem_3");


if __name__ == '__main__': 
    app = QtGui.QApplication(sys.argv)
    ex = Ui_SecDialog()        
    ex.show()
    sys.exit(app.exec_())