Question

Writing a script to render arbitrary HTML as an image, and using PyQt. I’m confused as to why my QApplication.exit call isn’t making QApplication.exec_ return. Any ideas on this?

class ScreenshotterApplication(QApplication):

    def __init__(self, args, html):
        QApplication.__init__(self, args)

        self.html = html

        self.browser = QWebView()
        self.browser.resize(0, 0)
        self.browser.loadFinished.connect(self.save)
        self.browser.loadProgress.connect(self.progress)

    def render(self):
        self.browser.setHtml(self.html)

    def progress(self, progress):
        print '%d%%' % progress

    def save(self, finished):
        success = False
        if finished:
            print 'saving...'
            # ... snip ...

            success = pixmap.save('screenshot.png')
            if success:
                print 'saved as "screenshot.png"'

        QApplication.exit(0 if success else 1)

    def exec_(self, *args, **kwargs):
        self.render()
        super(QApplication, self).exec_(*args, **kwargs)

def take_screenshot(html):
    app = ScreenshotterApplication(sys.argv, html)
    return app.exec_()

if __name__ == "__main__":
    print take_screenshot('<h1 style="width: 500px">Hello, World!</h1>')

Answer 1

QApplication.exit tells the application to leave the main event loop, i.e. to return from exec_() as soon as possible.

Your problem is that the small piece of static HTML is so easy to retrieve and render, QtWebKit does it right when setHtml is called. No delays or background processing, everything’s done before setHtml returns. So, save is called before the main loop starts (i.e. before exec_ is called).

It’s like writing return when you’re not in a function – except Qt will just ignore exit quietly if the loop isn’t running.

Solution: Use a QueuedConnection in your connect call to force the signal to be queued, and delivered when event loop starts. This’ll also work if the loop is already running, of course.

self.browser.loadFinished.connect(self.save, Qt.QueuedConnection)