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

[Dave]

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).
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