[FIXED] PySide program closes instantly

Issue

I'm trying to learn PySide for a project I'm working on. I'm working through the Zetcode tutorials, but from the very beginning I'm running problems. I am writing and running my code through Enthought's Canopy. When I run the code from the command line it works fine. This question may be related to my issue, however no answer is given there.

When I use the simplest code from the tutorial

import sys
from PySide import QtGui    

wid = QtGui.QWidget()
wid.resize(250, 150)
wid.setWindowTitle('Simple')
wid.show()

everything runs correctly. The next example does more or less the same, except from an OOP perspective.

import sys
from PySide import QtGui

class Example(QtGui.QWidget):

    def __init__(self):
        super(Example, self).__init__()

        self.initUI()

    def initUI(self):

        self.setGeometry(300, 300, 250, 150)
        self.setWindowTitle('Icon')
        self.setWindowIcon(QtGui.QIcon('web.png'))        

        self.show()

def main():

    ex = Example()    

if __name__ == '__main__':
    main()

When run the program flashes (I can see the window appear for a brief second) and then closes. Raising an exception before the end of main() will keep the window on the screen.

TL;DR

Why does putting the program in a class make it not work?

Solution

The difference between the two examples, is that first one keeps a reference to the widget as a global variable, whilst the second creates a local variable that gets garbage-collected when it goes out of scope (i.e. when the function returns).

The simplest way to fix this, is to make the ex variable global, like this:

def main():
    global ex
    ex = Example()

or you could just get rid of the main function, and simply do:

if __name__ == '__main__':        
     ex = Example()

Answered By - ekhumoro