运行脚本后pyqt4崩溃

Question

class Render(QtWebKit.QWebPage):
  def __init__(self, urls,app, cb):



    QWebPage.__init__(self)

    self.loadFinished.connect(self._loadFinished)

    self.urls = urls

    self.cb = cb

    self.app=app

    self.crawl()

    self.app.exec_()
    print(1)




  def crawl(self):
    if self.urls:

      url = self.urls.pop(0)

      print ('Downloading', url)

      self.mainFrame().load(QUrl(url))

    else:
      self.app.quit()
      print(2)



  def _loadFinished(self, result):
    frame = self.mainFrame()

    url = str(frame.url().toString())

    html = frame.toHtml()

    self.cb(url, html)

    self.crawl()
    class King(Resource):


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

第二次跑步后为什么会崩溃？