Python PyQT/PySide QThread limiting

Question

I have problem with thread limiting. I want to do it using QThread. So SpiderThread is QThread object crawling some urls. But I want to limit working threads to X threads at once. I have done it earlier with threadpool and QRunnable but it's buggy in pyside when numbers of urls are big. So I have this simple code:

 self.threads = []
    for url in self.urls:
        th = SpiderThread(url)
        th.updateresultsSignal.connect(self.update_results)
        self.threads.append(th)
        th.start()

Anyone have working example of limiting threads using QThread ?