closing a pyqt widget in ipython notebook without using sys.exit()

Question

I am trying to run through some pyqt5 tutorials in the ipython notebook, but have an issue where every second time I run a code block the kernal undergoes a forced restart.

Answer 1

I tried out Taar's solution but still got a dead kernel after calling the cell with main more than twice.The problem is creating multiple Qapplications, this crashes the notebook.

There is multiple solutions I found, but to just being able to run a qt application use the following in the first cell:

%gui qt
from PyQt5.QtWidgets import QApplication, QWidget

and in the second cell:

 if __name__ == '__main__':

        w = QWidget()
        w.setWindowTitle('Simple')
        w.show()

You can call the second cell as many times as you want and it will work. the magic line %gui qt opens a QApplication for your notebook.

If you need more control (like being able to exit() it) there is various solutions that amount to checking if there is a Qapplication instance open. Here is an example:

import sys
from PyQt5.QtWidgets import QApplication, QWidget
from PyQt5 import QtCore

second cell:

if __name__ == '__main__':

    app = QtCore.QCoreApplication.instance()
    if app is None:
        app = QApplication(sys.argv)

    w = QWidget()
    w.setWindowTitle('Simple')
    w.show()

    app.exec_()

This method does require closing the window before rerunning it (else they will be queued up: run 3x without closing the window, now you need to close the window 3x in a row). It will at least get you started with a properly loaded screen upon executing the cell. (anyone feel welcome to correct this example).

Some references for the second example: here, and here. But I don't know enough about how the qt gui interacts with the notebook to solve any problem with the above example.