def test_serializeSlots(self):
        """
        Test that flattening a slot will use the slot value from the tag.
        """
        t1 = tags.p(slot('test'))
        t2 = t1.clone()
        t2.fillSlots(test='hello, world')
        return gatherResults([
            self.assertFlatteningRaises(t1, UnfilledSlot),
            self.assertFlattensTo(t2, b'<p>hello, world</p>'),
        ])

        Test that flattening a slot will use the slot value from the tag.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_flatten.py