[PySide] PySide2 + multiprocessing + requests = crash?

[Jason H]

I've done something similar in there past. I would generate the images on the main thread, but then send it to a QThread/QRunnable for saving because I was using PNG and the save took far longer than anything else. All my cores were saturated with saves and the QImage generation was 100s of frames ahead. Fortunately, I had enough RAM to render the sequence. 

Maybe something like that would work better? But I did it all in C++ so no GIL. 

> Sent: Tuesday, December 17, 2019 at 8:06 PM
> From: "Israel Brewster" <ijbrewster at alaska.edu>
> To: "Jason H" <jhihn at gmx.com>
> Cc: pyside at qt-project.org
> Subject: Re: [PySide] PySide2 + multiprocessing + requests = crash?
>
> 
> > On Dec 17, 2019, at 9:56 AM, Jason H <jhihn at gmx.com> wrote:
> > 
> > QApplication has to be on the main thread.
> > So I would guess (without seeing the crash) that yes, it's bad form. 
> 
> It *is* on the main thread. Of a child process, true, but the main thread of that process. Multiprocessing, not multithreading. Presumably that’s why it generally works - as long as I don’t try to make a web request :-)
> 
> >  
> > Generally when I want to do something like ehat you are doing, I set a singlshot timer to fire (interval: 0) start the event loop (app.exec() or app.exec_() in python) then the event loop wth execute the timer code.
> 
> No event loop here. I’m just creating QWidgets, setting contents, and using .grab() to save the contents as an image, after which the widgets are discarded. The only reason I have a QApplication at all is that you can’t create QWidgets without one.
> 
> There is no user interaction, no displayed GUI at any point. In fact, I’m initializing QApplication with a platform of offscreen. I just want to work with multiple QWidgets simultaneously to generate multiple images in parallel - thus the need for multiprocessing.
> 
> >  
> >  
> >  
> > Sent: Tuesday, December 17, 2019 at 1:13 PM
> > From: "Israel Brewster" <ijbrewster at alaska.edu>
> > To: pyside at qt-project.org
> > Subject: [PySide] PySide2 + multiprocessing + requests = crash?
> > I’ve found what seems to be an odd crash to me. Using PySide2 and multiprocessing works fine to create a QAppliction in a separate process (unless this is considered bad form?) - unless I call requests.get at some point before starting said process, in which case the process crashes (hard crash, not python exception). How might I fix this?
> >  
> > A little background: I’m using Qt classes to generate a bunch of images from data files. To speed things along, I launch a process (using multiprocessing.Process) for each image I want to generate, and in each process I create a QApplication, create and fill the widgets I need for the image, and then save the image to a file. This appears to work fine, creating the images in parallel. Today I tried to do a requests call before doing any of the processing (simple get, doesn’t interact with QT in any way), and suddenly my code started crashing.
> >  
> > Some simplified sample code that illustrates the issue:
> >  
> > import time
> > import multiprocessing
> > import sys
> > from PySide2.QtWidgets import QApplication
> > import requests
> >  
> >  
> > def test_wait():
> >     print("Starting proc")
> >     try:
> >         QApplication(sys.argv + ['-platform', 'offscreen'])
> >     except Exception as err:
> >         print("oops", err)
> >  
> >     print("Sleeping now...")
> >     time.sleep(10)
> >  
> > if __name__ == "__main__":
> >     proc = multiprocessing.Process(target=test_wait)
> >     proc.start()
> >     proc.join()
> >     print("Awake again. Let's try another:")
> >  
> >     res = requests.get('https://www.google.com <https://www.google.com/>')  # or whatever your favorite is
> >     res.text  # Get the response, close it out
> >  
> >     proc = multiprocessing.Process(target=test_wait)
> >     start = time.time()
> >     proc.start()
> >     proc.join()
> >  
> >     # The process should sleep for 10 seconds, so if we got here in less than 9, we died.
> >     if time.time() - start < 9:
> >         print("Never slept :(")
> >     else:
> >         print("We lived!")
> >  
> > ---
> > Israel Brewster
> > Software Engineer
> > Alaska Volcano Observatory 
> > Geophysical Institute - UAF 
> > 2156 Koyukuk Drive 
> > Fairbanks AK 99775-7320
> > Work: 907-474-5172
> > cell:  907-328-9145
> > _______________________________________________ PySide mailing list PySide at qt-project.org https://lists.qt-project.org/listinfo/pyside <https://lists.qt-project.org/listinfo/pyside>
>