[PYTHON] A solution to the problem that kernel restarting frequently occurs when running PyQt system with jupyter or Spyder IDE

Even when doing trial and error with a GUI program such as PyQt or PyQtGraph I often use jupyter etc. because it is convenient, There is one of the most annoying points.

When I run a PyQt GUI program several times,

** The kernel will definitely die. ** **

I finally found a workaround.

It didn't matter if sys.exit () was present at the end.

This → sys.exit (app.exec_ ())

I thought it was because sys.exit ended strangely, but it was innocent. The presence or absence of sys.exit () at exit was irrelevant (at least in jupyter). (Spyder is unconfirmed)

Just overwrite the app with something else just before! (PyQt4 only. Cannot be used with PySide)

app = 0 # Add this line app = QtGui.QApplication(sys.argv)

With this alone, even if you run the same code with jupyter many times, The annoying kernel restart is gone! Yay!

reference https://www.reddit.com/r/learnpython/comments/45h05k/solved_kernel_crashing_when_closing_gui_spyder/

2016/09/01 postscript

I could do it with mojaie's method in the comment. No matter how you think about it, this method is right.

app = QtGui.QApplication.instance()
if app is None:
    app = QtGui.QApplication(()) 

Also, while PyQt4 runs silently and the kernel dies, I was shocked that PySide prevented the creation of multiple instances of QApplication in advance and returned a runtime error. It's the difference between PyQt4 and PySide. .. ..

Also, PySide causes a runtime error, so app = 0 cannot be used. app = 0 is a magic for PyQt4 ...