PyQt5 program runs but does not display anything - widgets does not show [duplicate] - python-3.x

Question

First of all, similar questions have been answered before, yet I need some help with this one.
I have a window which contains one button (Class First) and I want on pressed, a second blank window to be appeared (Class Second).
I fiddled with the code copied from this question: PyQT on click open new window, and I wrote this code:
# -*- coding: utf-8 -*-
from PyQt4 import QtGui, QtCore
import sys
import design1, design2
class Second(QtGui.QMainWindow, design2.Ui_MainWindow):
def __init__(self, parent=None):
super(Second, self).__init__(parent)
self.setupUi(self)
class First(QtGui.QMainWindow, design1.Ui_MainWindow):
def __init__(self, parent=None):
super(First, self).__init__(parent)
self.setupUi(self)
self.pushButton.clicked.connect(self.on_pushButton_clicked)
self.dialog = Second(self)
def on_pushButton_clicked(self):
self.dialog.exec_()
def main():
app = QtGui.QApplication(sys.argv)
main = First()
main.show()
sys.exit(app.exec_())
if __name__ == '__main__':
main()
but on_pressed, this error message appears:
AttributeError: 'Second' object has no attribute 'exec_'
(design1 and design2 have been derived from the Qt designer.)
Any thought would be appreciated.

Answer 1

Here I'm using the show method.
Here is a working example (derived from yours):
#!/usr/bin/env python
# -*- coding: utf-8 -*-
from PyQt4 import QtGui, QtCore
import sys
class Second(QtGui.QMainWindow):
def __init__(self, parent=None):
super(Second, self).__init__(parent)
class First(QtGui.QMainWindow):
def __init__(self, parent=None):
super(First, self).__init__(parent)
self.pushButton = QtGui.QPushButton("click me")
self.setCentralWidget(self.pushButton)
self.pushButton.clicked.connect(self.on_pushButton_clicked)
self.dialog = Second(self)
def on_pushButton_clicked(self):
self.dialog.show()
def main():
app = QtGui.QApplication(sys.argv)
main = First()
main.show()
sys.exit(app.exec_())
if __name__ == '__main__':
main()
If you need a new window every time you click the button, you can change the code that the dialog is created inside the on_pushButton_clicked method, like so:
#!/usr/bin/env python
# -*- coding: utf-8 -*-
from PyQt4 import QtGui, QtCore
import sys
class Second(QtGui.QMainWindow):
def __init__(self, parent=None):
super(Second, self).__init__(parent)
class First(QtGui.QMainWindow):
def __init__(self, parent=None):
super(First, self).__init__(parent)
self.pushButton = QtGui.QPushButton("click me")
self.setCentralWidget(self.pushButton)
self.pushButton.clicked.connect(self.on_pushButton_clicked)
self.dialogs = list()
def on_pushButton_clicked(self):
dialog = Second(self)
self.dialogs.append(dialog)
dialog.show()
def main():
app = QtGui.QApplication(sys.argv)
main = First()
main.show()
sys.exit(app.exec_())
if __name__ == '__main__':
main()