def test_callbackContext(self):
        """
        The context L{ThreadPool.callInThreadWithCallback} is invoked in is
        shared by the context the callable and C{onResult} callback are
        invoked in.
        """
        myctx = context.theContextTracker.currentContext().contexts[-1]
        myctx['testing'] = 'this must be present'

        contexts = []

        event = threading.Event()

        def onResult(success, result):
            ctx = context.theContextTracker.currentContext().contexts[-1]
            contexts.append(ctx)
            event.set()

        def func():
            ctx = context.theContextTracker.currentContext().contexts[-1]
            contexts.append(ctx)

        tp = threadpool.ThreadPool(0, 1)
        tp.callInThreadWithCallback(onResult, func)
        tp.start()
        self.addCleanup(tp.stop)

        event.wait(self.getTimeout())

        self.assertEqual(len(contexts), 2)
        self.assertEqual(myctx, contexts[0])
        self.assertEqual(myctx, contexts[1])

        The context L{ThreadPool.callInThreadWithCallback} is invoked in is
        shared by the context the callable and C{onResult} callback are
        invoked in.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_threadpool.py