def test_deferSetMultipleTimes(self):
        """
        L{crefutil._Defer} can be assigned a key only one time.
        """
        d = crefutil._Defer()
        d[0] = 1
        self.assertRaises(RuntimeError, d.__setitem__, 0, 1)

        L{crefutil._Defer} can be assigned a key only one time.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_persisted.py