Method __setitem__

Lib/test/test_copy.py:735–738

(self, key, item)

                self._keys = list(d.keys())
                super().__init__(d)
            def __setitem__(self, key, item):
                super().__setitem__(key, item)
                if key not in self._keys:
                    self._keys.append(key)
        x = C(d={'foo':0})
        y = copy.deepcopy(x)
        self.assertEqual(x, y)