simple IPython example raises exception on sys.exit()

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This answer is thanks to Matthias BUSSONNIER from the ipython-users mailing list.

When I close the window of the running demo application, I see this error:
An exception has occurred, use %tb to see the full traceback.
SystemExit: 0

Just don’t use sys.exit(0) as you are not exiting python, but still running IPython.

Add it back if you wish to run your app from a (real) command line and have a return status.

If I try to execute my code again, I get this:
RuntimeError: A QApplication instance already exists.

This is a PySide Bug that they “won’t fix” as they don’t consider it a bug.

See https://github.com/ipython/ipython/issues/1124)
and http://bugs.pyside.org/show_bug.cgi?id=855

QApplication can only have one instance and quitting an app is
apparently not considered a reason sufficient enough do delete the
object…

You can use this code from above issues :

app=QtGui.QApplication.instance() # checks if QApplication already exists 
if not app: # create QApplication if it doesnt exist 
     app = QtGui.QApplication(sys.argv)

This was a sufficient solution for my present needs.

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