pyqt4 - simple IPython example raises exception on sys.exit()

Question

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

If I try to execute my code again, I get this:

RuntimeError: A QApplication instance already exists.

In case it helps, here my code:

from PySide.QtCore import *
from PySide.QtGui import *
import sys

class MyWindow(QWidget):
    def __init__(self):
        QWidget.__init__(self, None)

        vbox = QVBoxLayout(self)

        self.slider1 = QSlider(Qt.Horizontal)
        self.slider1.setRange(0, 99)
        self.slider1.setValue(0)
        vbox.addWidget(self.slider1)

        self.slider2 = QSlider(Qt.Horizontal)
        self.slider2.setRange(0, 99)
        self.slider2.setValue(99)
        vbox.addWidget(self.slider2)

        self.slider1.valueChanged.connect(self.slider2Changed)

    def slider2Changed(self, position):
        self.slider2.setValue(self.slider2.maximum() - position)

def main():
        app = QApplication(sys.argv)
        w = MyWindow()
        w.show()
        app.exec_()
        sys.exit(0)

if __name__ == "__main__":
    main()

I do not have errors when running the code using python:

python myexample.py

This error only happens when I run the code in an IPython (including a notebook or the qtconsole or the regular ipython terminal).

UPDATE: My main problem is that I cannot run the application again quickly and easily. If I try to run my code again, I get this:

RuntimeError: A QApplication instance already exists.

That kills the fast, interactive nature of IPython :(

Answer 1

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.