PyQT: how to open new window

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()