Question

Basically I have a main window with a menu bar and a number of options . When I click on one of the options , my code should open another window. My code looks something like this now.
All required libraries are imported.

class subwindow(self):
    //Another small window

class MainWindow(QtGui.QMainWindow):
    def __init__(self):
        super(MainWindow , self).__init__()     
        self.window()

    def window(self):
         Action = QtGui.QAction(QtGui.QIcon('action.png') , 'action' , self)          
         Action.triggered.connect(self.a)

         mb = self.menuBar()
         option = mb.addMenu('File')
         option.addAction(Action)

         self.show()

   def a(self):
         s = subwindow()



if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    mw = MainWindow()
    sys.exit(app.exec_())

How do I run the sub-window part of the code.How do I add the QtGui.QApplication part?

Answer 1

Like @Bakuriu said in the comments, there must be only one instance of QApplication. This starts the main event loop for the application.

You can create a new window by deriving your SubWindow class from the QDialog class, and customizing it as you want.
You need to call the exec_() method of the QDialog class to get the dialog to show.

For example, in your code:

from PyQt4 import QtGui
import sys

class SubWindow(QtGui.QDialog):
    def __init__(self):
        super(SubWindow , self).__init__()     
        label = QtGui.QLabel("Hey, subwindow here!",self);

class MainWindow(QtGui.QMainWindow):
    def __init__(self):
        super(MainWindow , self).__init__()     
        self.window()

    def window(self):
        Action = QtGui.QAction(QtGui.QIcon('action.png') , 'action' , self)          
        Action.triggered.connect(self.a)

        mb = self.menuBar()
        option = mb.addMenu('File')
        option.addAction(Action)

        self.show()

    def a(self):

        s = SubWindow()
        s.exec_()

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    mw = MainWindow()
    sys.exit(app.exec_())