[IPython-User] simple iPython example raises exception on sys.exit()

[Matthias BUSSONNIER]

Hi, 

Le 4 juin 2012 à 23:24, Dave a écrit :

> I'm doing some very simple PySide (and PyQt) tutorials in iPython. One tutorial just creates a window with some sliders to demonstrate slots and signals.
> 
> 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
> To exit: use 'exit', 'quit', or Ctrl-D.
> So I run %tb and get this:
> 
> SystemExit                                Traceback (most recent call last)
> /Workspaces/scratch/<ipython-input-1-88966dcfb499> in <module>()
>      33 
>      34 if __name__ == "__main__":
> ---> 35     main()
> 
> /Workspaces/scratch/<ipython-input-1-88966dcfb499> in main()
>      29         w.show()
>      30         app.exec_()
> ---> 31         sys.exit(0)
>      32 
>      33 
> 
> SystemExit: 0

Just don't use sys.exit(0) as you are not exiting python, but still running IPython.
Add it 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)
QApplication can only have one instance and quitting an app is apparently not considered a resin 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)
….

to reuse the application instance, it might have a few side effect, (not sure) but you shouldn't be confronted to it if you are starting with qt. 

Happy coding,
-- 
Matthias

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