How to use the greenlet.greenlet.error function in greenlet

def test_switch_to_another_thread(self):
        data = {}
        error = None
        created_event = threading.Event()
        done_event = threading.Event()

        def foo():
            data['g'] = greenlet(lambda: None)
            created_event.set()
            done_event.wait()
        thread = threading.Thread(target=foo)
        thread.start()
        created_event.wait()
        try:
            data['g'].switch()
        except greenlet.error:
            error = sys.exc_info()[1]
        self.assertTrue(error != None, "greenlet.error was not raised!")
        done_event.set()
        thread.join()

def test_unexpected_reparenting(self):
        another = []
        def worker():
            g = greenlet(lambda: None)
            another.append(g)
            g.switch()
        t = threading.Thread(target=worker)
        t.start()
        t.join()
        class convoluted(greenlet):
            def __getattribute__(self, name):
                if name == 'run':
                    self.parent = another[0]
                return greenlet.__getattribute__(self, name)
        g = convoluted(lambda: None)
        self.assertRaises(greenlet.error, g.switch)