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class MainWindow(QMainWindow):
    
    def __init__(self, parent=None):
        QWidget.__init__(self, parent)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)
        
        self.ui.pushButton1.clicked.connect(self.on_Button_1clicked)
    
    def on_Button_1clicked(self):
        dialog = Dialog1()
        QMainWindow.hide()
        ret=dialog.exec()    
        print(ret)
 
 
class Dialog1(QDialog):
    
    def __init__(self):
        QDialog.__init__(self)
        self.ui = Ui_Dialog1()
        self.ui.setupUi(self)

import sys
 
from PyQt4 import QtGui
 
 
class MainWindow(QtGui.QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.secondWin = None
        self.build()
 
    def build(self):
        self.mainLayout = QtGui.QVBoxLayout()
 
        self.lab = QtGui.QLabel('simple text', self)
        self.mainLayout.addWidget(self.lab)
 
        self.but1 = QtGui.QPushButton('open window', self)
        self.but1.clicked.connect(self.open_win)
        self.mainLayout.addWidget(self.but1)
 
        self.setLayout(self.mainLayout)
 
    def open_win(self):
        self.secondWin = QtGui.QPushButton('some text')
        self.secondWin.show()
        self.hide()
 
 
if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    window = MainWindow()
    window.show()
    sys.exit(app.exec_())