Exceptions in PyQt event loop and ipython · Issue #10057 · ipython/ipython (original) (raw)

I am not sure if this is really a bug, but rather an issue that I am facing with ipython and option qt. My problem was also posted 2 days ago on Stackoverflow (http://stackoverflow.com/questions/40608610/exceptions-in-pyqt-event-loop-and-ipython). If the problem turns out to be solvable, ideally, it would be nice to have a quick fix for now (which function to monkey patch?). Then, maybe a more permanent fix can be included in future releases.

I have a PyQt program that is displaying some widgets and buttons.

I want the program to run either as a standalone python instance, or inside an ipython environment. In this case, I use the following magic command in Jupyter console (formerly I had to use --gui=qt when launching the ipython qtconsole)

In order to have a program that works both ways, my main module has the following lines:

APP = QtGui.Qapplication.instance() # retrieves the ipython qt application if any
if APP is None:
    APP = QtGui.QApplication(["foo"]) # create one if standalone execution

if __name__=='__main__':
    APP.exec_() # Launch the event loop here in standalone mode 

Here is my problem: exceptions generated by the event loop are very hard to detect by the user because they pop-out in the background console. I would like to catch any exception occuring in the event loop, and display a warning (for intance in the QMainWindow status bar to make the user aware that an exception occured).

I have tried several strategies, but there seems to be a conspiracy between PyQt's and Ipython's internal machineries to make this impossible:

• Reimplement sys.excepthook (see http://stackoverflow.com/questions/18740884/preventing-pyqt-to-silence-exceptions-occurring-in-slots): not working because ipython keeps overwritting sys.excepthook
• Detecting if IPython is running, and then using IPYTHON.set_custom_exc (http://stackoverflow.com/questions/15752437/opening-an-ipython-shell-on-any-uncatched-exception): Unforunately, qt event loop exceptions do not trigger the handler.
• Overwrite QApplication.notify: bad luck, the native QApplication.notify function that I intend to call in the derived function is not throwing exceptions, nor does the return value (boolean) reflect the correct execution of the slots. The answer in this thread is interesting: http://stackoverflow.com/questions/10665655/how-to-log-uncatched-exceptions-of-a-qapplication, however, it seems that this strategy works in Qt c++, but the python wrapper of notify just prints the exceptions to the console instead of raising them.
• Overwriting the function that IPython will use as a monkey patch for sys.excepthook (as suggested in http://stackoverflow.com/questions/1261668/cannot-override-sys-excepthook/28758396#28758396) is not working either. In fact, Ipython with the qt option will prevent the qt event loop from even calling sys.excepthook.

It is a problem that keeps bothering me since a long time. Does anyone have a solution?