import sys
from MAIN import Ui_MainWindow
from PyQt5 import QtCore, QtGui,QtWidgets
from dialog3 import Ui_Dialog3
class Secwin(QtWidgets.QMainWindow):
    def __init__(self):
        QtWidgets.QMainWindow.__init__(self)
        self.ui = Ui_Dialog3()
        self.ui.setupUi(self)
            
class main(QtWidgets.QMainWindow,Ui_MainWindow):
    def __init__(self):
        QtWidgets.QMainWindow.__init__(self)
        self.setupUi(self)
        self.pushButton.clicked.connect(self.open_win)
    def  open_win(self):
        child=Secwin()
        child.show()
            
if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    myapp = main()
    myapp.show()
    sys.exit(app.exec_())
        
        

 from PyQt4.Qt import *
class Dialog(QDialog):
    def __init__(self):
        QDialog.__init__(self)
        self.Label = QLabel("Dialog",self)
        self.resize(300,100)
class Window(QWidget):
    def __init__(self):
        QWidget.__init__(self)
        self.btn = QPushButton("Push Me",self)
        self.btn.clicked.connect(self.on_btn)
    def on_btn(self):
        dlg = Dialog()
        ret = dlg.exec_()
        print ret
 
app = QApplication([])
w = Window()
w.show()
app.exec_()