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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Incorrect value for `Py_TPFLAGS_MANAGED_DICT` in `Lib/test/test_class.py`","articleBody":"I've noticed a discrepancy in the value of the `Py_TPFLAGS_MANAGED_DICT` flag constant used in `Lib/test/test_class.py`.\n\n### Summary\n\nThe test file `Lib/test/test_class.py` defines a local constant `Py_TPFLAGS_MANAGED_DICT = (1 \u003c\u003c 2)`. However, the canonical definition in the C header `Include/object.h` is `Py_TPFLAGS_MANAGED_DICT = (1 \u003c\u003c 4)`.\n\nThe existing tests that use this constant pass only coincidentally. They test plain classes which happen to have a flag set at the `(1 \u003c\u003c 2)` position, but this is not the actual managed dictionary flag.\n\nThis can be proven by a test case that uses a `__slots__` class. Such a class should not have a managed dictionary, and therefore, a test checking for the `MANAGED_DICT` flag should result in `0`. However, the test written to check for the incorrect `(1 \u003c\u003c 2)` flag also results in `0`, which causes an assertion to fail, proving the constant is wrong.\n\n### Proof of Incorrect Constant\n\nThe following test case demonstrates that the `(1 \u003c\u003c 2)` constant is incorrect. It uses a `__slots__` class, which is known not to have a managed dictionary.\n\nThe test attempts to assert that this class *has* the flag specified by the incorrect `(1 \u003c\u003c 2)` constant. This assertion fails, providing evidence that the constant is wrong.\n\n```python\n# In Lib/test/test_class.py\nimport unittest\n\nclass TestMisleadingConstant(unittest.TestCase):\n    def test_slots_class_reveals_flaw_in_constant(self):\n        # The incorrect constant as defined in test_class.py\n        Py_TPFLAGS_MANAGED_DICT_WRONG = (1 \u003c\u003c 2)\n        # The correct constant from object.h\n        Py_TPFLAGS_MANAGED_DICT_CORRECT = (1 \u003c\u003c 4)\n\n        class NoManagedDict:\n            __slots__ = ('a',)\n\n        # First, confirm that a __slots__ class correctly does NOT have\n        # the true managed dict flag. This assertion passes.\n        self.assertEqual(\n            NoManagedDict.__flags__ \u0026 Py_TPFLAGS_MANAGED_DICT_CORRECT,\n            0,\n            \"A __slots__ class should not have the correct MANAGED_DICT flag (1\u003c\u003c4)\"\n        )\n\n        # Next, try to assert that this class HAS the flag at the (1\u003c\u003c2) position.\n        # This assertion FAILS with \"AssertionError: 0 != 4\", because a __slots__ class\n        # does not have this flag either.\n        # This failure is the proof that the (1\u003c\u003c2) constant is wrong for this purpose.\n        self.assertEqual(\n            NoManagedDict.__flags__ \u0026 Py_TPFLAGS_MANAGED_DICT_WRONG,\n            Py_TPFLAGS_MANAGED_DICT_WRONG,\n            \"This fails, proving the constant is wrong.\"\n        )\n```\n\n### Reference Links\n\n1.  **Incorrect constant in `test_class.py`**:\n    *   https://github.com/python/cpython/blob/49365bd110a254a6a304d2f880482bfeb2e4490d/Lib/test/test_class.py#L862\n\n2.  **Correct constant definition in `object.h`**:\n    *   https://github.com/python/cpython/blob/49365bd110a254a6a304d2f880482bfeb2e4490d/Include/object.h#L532-L535\n\n### Proposed Fix\n\nThe constant `Py_TPFLAGS_MANAGED_DICT` in `Lib/test/test_class.py` should be updated to `(1 \u003c\u003c 4)` to match the C header definition.\nAdditionally, we could add a test case for a class that does not have `Py_TPFLAGS_INLINE_VALUES`.\n\nIf this analysis seems correct, I would be happy to open a Pull Request to update the constant and add a corresponding test case. \n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-136538\n* gh-140532\n* gh-140533\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ever0de","@type":"Person","name":"ever0de"},"datePublished":"2025-07-11T10:10:55.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/136535/cpython/issues/136535"}