QThreadPool QRuanable signals workaround

import sys
import urllib2

from PyQt4 import QtCore, QtGui


class SignalHelper(QtCore.QObject):
    data_downloaded = QtCore.pyqtSignal(object)


class DownloadThread(QtCore.QRunnable):
    def __init__(self, url):
        QtCore.QRunnable.__init__(self)
        self.url = url
        self._signal_helper = SignalHelper()
        self.data_downloaded = self._signal_helper.data_downloaded

    def run(self):
        print 'thread', QtCore.QThread.currentThread()
        info = urllib2.urlopen(self.url).info()
        self.data_downloaded.emit('%s\n%s' % (self.url, info))


class MainWindow(QtGui.QWidget):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.list_widget = QtGui.QListWidget()
        self.button = QtGui.QPushButton("Start")
        self.button.clicked.connect(self.start_download)
        layout = QtGui.QVBoxLayout()
        layout.addWidget(self.button)
        layout.addWidget(self.list_widget)
        self.setLayout(layout)
        #
        ## Thread pool with maximum 2 simultaneously running threads
        #
        self.thread_pool = QtCore.QThreadPool()
        self.thread_pool.setMaxThreadCount(2)

    def start_download(self):
        urls = ['http://google.com', 'http://twitter.com', 'http://yandex.ru',
                'http://stackoverflow.com/', 'http://www.youtube.com/']
        for url in urls:
            downloader = DownloadThread(url)
            downloader.data_downloaded.connect(self.on_data_ready)
            self.thread_pool.start(downloader)

    def on_data_ready(self, data):
        self.list_widget.addItem(data)



if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    window = MainWindow()
    window.resize(640, 480)
    window.show()
    sys.exit(app.exec_())