how to open a Dialog window on top of my mainwindow

When I try go show a window (AddUserGui) it just resize the Ui_MainWindow. I think the problem occurs when I send "self" as a argument. How do you solve this? or is this a stuipd way to do it?

Code:

---

from PyQt4 import QtCore, QtGui
from libarylib import *
from bibgui import Ui_MainWindow
from addusergui import Ui_Dialog as AddUserGui
import sys
 
 
class mainGuiWindow(QtGui.QMainWindow):
 
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)
 
        self.connect(self.ui.pushButton_3, QtCore.SIGNAL("clicked()"), self.addUser)
 
 
 
    def addUser(self):
        self.adduser = AddUserGui()
        self.adduser.setupUi(self)              #<-- this don't work
 
 
 
 
 
 
 
 
if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    myapp = mainGuiWindow()
    myapp.show()
    sys.exit(app.exec_())

---