QApplication (PySide) Django app issue with quitting

Question

I have a Django app in which the users can create reports at various places throughout the webapp. As a little nifty feature I would like to make a "send as PDF" feature for these reports, and I'm almost there.

What I do is that before the report is returned as a HttpResponse through Django I send the raw HTML content through a small PySide/QT snippet (as seen below).

The problem is that I can't get the QApplication to quit.

I have tried with the standard QCoreApplication.exit() but with no luck. If I try to convert a new report right after the first one, the console says a "QApplication instance already exists".

I am using Django 1.2.5, Python 2.7, QT 4.8 and PySide 1.1 on OS X 10.7.3 (for testing).

Code:

def makepdf(response,filename):
    try:
        app = QApplication(sys.argv)
    except:
        app = QCoreApplication.instance()
    web = QWebView()

    stylelink = "%s%s" % (media_root,'images/css/reportGenerator.css') 

    web.settings().setUserStyleSheetUrl(QUrl.fromLocalFile(stylelink))
    web.setHtml(response)

    printer = QPrinter()
    printer.setPageSize(QPrinter.A4)
    printer.setOutputFormat(QPrinter.PdfFormat)
    printer.setOutputFileName(filename)

    def convertToPdf():
        web.print_(printer)
        #webresponse = web.close()        
        #QObject.disconnect()
        #print QCoreApplication.instance().children()[0].interrupt()
        #qthread = QCoreApplication.instance().thread()#.cleanup()
        #qthread.exit()
        QCoreApplication.exit()
    QObject.connect(web, SIGNAL("loadFinished(bool)"), convertToPdf())

Code comments:

Currently I have made a try/except clause to be able to keep the code running, by using the current QApplication instance (to avoid the 'instance exists error'), but this just doesn't seem right?. I mean, having a QAppliation running for the duration of my Apache server (which will be the case in production) seems a bit off. Shouldn't it be possible to quit it after the PDF conversion is done?

Other than the QCoreApplication.exit() I have tried to use the sys.exit(app.exec_()) method, as is often seen in examples and snippets. This however just causes an error and makes Python crash and the code works perfectly without this. (well it creates the PDF, but won't quit).

All the lines commented out are the previous attempts I have made to quit the QApplication.

In short: I don't know what it is, but it just won't quit. Can anyone suggest why?

Update: After the latest answer I have edited the code to respond to the input. This is how the final part looks now:

def convertToPdf():
    web.print_(printer)
    app.exit()
web.loadFinished.connect(convertToPdf)
app.exec_()

I do, however, still get an error:

2012-04-30 00:16:10.791 Python[21241:1803] * Assertion failure in +[NSUndoManager _endTopLevelGroupings], /SourceCache/Foundation/Foundation-833.24/Misc.subproj/NSUndoManager.m:324
Qt has caught an exception thrown from an event handler. Throwing exceptions from an event handler is not supported in Qt. You must reimplement QApplication::notify() and catch all exceptions there.

This error only occurs when I implement app.exec_(). However without app.exec_() it is just the normal issue again, with no quitting.

Any ideas?

Update 2: this is the newest code, fixed in accordance with matas latest suggestion:

app = QApplication(sys.argv)
web = QWebView()
printer = QPrinter()
printer.setPageSize(QPrinter.A4)
printer.setOutputFormat(QPrinter.PdfFormat)
printer.setOutputFileName(filename)
def convertToPdf():
    web.print_(printer)
    app.exit()
web.loadFinished.connect(convertToPdf)
web.setHtml(response)

I still have the same problem however.

Answer 1

what i can say is this:

QObject.connect(web, SIGNAL("loadFinished(bool)"), convertToPdf())

here you call convertToPdf(), if you want to connect the signal, omit the parenthesis!

you could also use this much clearer syntax:

web.loadFinished.connect(convertToPdf)

you may also want to add a parameter to convertToPdf, as it is called with a boolean indicating wheather loading was successful or not.

And using app.exit() should be enough.

oh, and when you use Gui-Components you need to use a QApplication. A QCoreApplication won't do!

edit: it's important to call web.setHtml after you've connected the loadFinished signal! otherwise if the loading already is finished, your function will never be executed!

edit: this works without any problem for me:

#!/usr/bin/env python
from PySide.QtGui import *
from PySide.QtWebKit import *
import sys
from subprocess import Popen, PIPE


def createPdf(html, filename):
    app = QApplication(sys.argv)
    web = QWebView()
    printer = QPrinter()
    printer.setPageSize(QPrinter.A4)
    printer.setOutputFormat(QPrinter.PdfFormat)
    printer.setOutputFileName(filename)
    def convertToPdf():
        web.print_(printer)
        app.exit()
        app.deleteLater()
    web.loadFinished.connect(convertToPdf)
    web.setHtml(html)
    app.processEvents()

def createPdfInSubprocess(html, filename):
       p = Popen(["python", __file__, filename], 
                stdin=PIPE, stdout=PIPE, stderr=PIPE)
       out, err = p.communicate(html)
       return (p.returncode, out, err)

if __name__ == '__main__':
    if len(sys.argv) > 1:
        # read html from stdin, filename from cmdline
        html = "\n".join(sys.stdin.readlines())
        createPdf(html, sys.argv[1])
        # print("done")
    else:
        # test if it's working
        ret = createPdfInSubprocess(
                "<html><h1>test</h1>it's working...</html>", "test.pdf")
        print(ret)

Even without all the calls to app.exit(), app.deleteLater(), app.processEvents(), it still works... but can't hurt to have that.

One more important thing: QApplications have to be created from the main thread! So if it's running inside a django app it's probably not going to work, that's why i added the subprocess stuff...

Answer 2

PySide is designed such that there is only ever one instance of QCoreApplication within a process. The best reference to this (possibly undocumented fact) that I could find was http://bugs.pyside.org/show_bug.cgi?id=855.

Basically, there may be dangling references to the qapplication that prevent it from being reaped by the garbage collector, so even if you tell the application to exit and you delete your reference, there may still be other references lying around.