I can not open QDialog

I'm doing a QMainWindow that calls a QDialog , when executing the QDialog it opens without problems but when placing the code so that it opens from the QMainWindow by clicking on the button it throws me the following error >

C:\Users\marco\Downloads\Python\PyQt\Condominio>python menuppal.py
Traceback (most recent call last):
  File "menuppal.py", line 29, in dialogo
    dialog = Dialog()
NameError: name 'Dialog' is not defined

This is the code of the main window:

# -*- coding: utf-8 -*-

# Form implementation generated from reading ui file 'C:\Users\marco\Downloads\Python\PyQt\condominio\menuppal.ui'
#
# Created by: PyQt4 UI code generator 4.11.4
#
# WARNING! All changes made in this file will be lost!

from PyQt4 import QtCore, QtGui
from listageneral import *
try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    def _fromUtf8(s):
        return s

try:
    _encoding = QtGui.QApplication.UnicodeUTF8
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig, _encoding)
except AttributeError:
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig)

class Ui_MainWindow(object):

    def dialogo(self, checked=None):
        if checked==None: return
        dialog = Dialog()
        dialog.ui = Ui_Dialog()
        dialog.ui.setupUi(dialog)
        dialog.setAttribute(QtCore.Qt.WA_DeleteOnClose)
        dialog.exec_()

    def setupUi(self, MainWindow):
        MainWindow.setObjectName(_fromUtf8("MainWindow"))
        MainWindow.resize(714, 600)
        self.centralwidget = QtGui.QWidget(MainWindow)
        self.centralwidget.setObjectName(_fromUtf8("centralwidget"))
        self.tabWidget = QtGui.QTabWidget(self.centralwidget)
        self.tabWidget.setGeometry(QtCore.QRect(20, 40, 661, 401))
        self.tabWidget.setObjectName(_fromUtf8("tabWidget"))
        self.tab = QtGui.QWidget()
        self.tab.setObjectName(_fromUtf8("tab"))
        self.listgen = QtGui.QPushButton(self.tab)
        self.listgen.setGeometry(QtCore.QRect(30, 60, 75, 23))
        self.listgen.setObjectName(_fromUtf8("listgen"))
        self.listgen.clicked.connect(self.dialogo)      
        self.tabWidget.addTab(self.tab, _fromUtf8(""))
        self.tab_2 = QtGui.QWidget()
        self.tab_2.setObjectName(_fromUtf8("tab_2"))
        self.tabWidget.addTab(self.tab_2, _fromUtf8(""))
        MainWindow.setCentralWidget(self.centralwidget)
        self.statusbar = QtGui.QStatusBar(MainWindow)
        self.statusbar.setObjectName(_fromUtf8("statusbar"))
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        self.tabWidget.setCurrentIndex(0)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow", None))
        self.listgen.setText(_translate("MainWindow", "Lista General", None))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab), _translate("MainWindow", "Consulta", None))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab_2), _translate("MainWindow", "Tab 2", None))


if __name__ == "__main__":
    import sys
    app = QtGui.QApplication(sys.argv)
    MainWindow = QtGui.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())