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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Use Py_T_OBJECT_EX instead of _Py_T_OBJECT","articleBody":"`_Py_T_OBJECT` is considered legacy PyMemberDef type. The difference between `_Py_T_OBJECT` and `Py_T_OBJECT_EX` is that the former returns `None` if read `NULL`, while the latter raises AttrributeError. `_Py_T_OBJECT` manifests itself in two effects:\r\n\r\n1. The default value of the attribute is `None`, even if it was not initialized in the constructor. It is a desirable behavior in some cases.\r\n2. After deleting an attribute its value is still `None`. You cannot truly delete it.\r\n\r\nA `Py_T_OBJECT_EX` member behaves like a normal attribute in Python object, while a `_Py_T_OBJECT` member behaves like in the case when the corresponding class attribute was set to `None`:\r\n\r\n```python\r\nclass A:\r\n    attr = None\r\n\r\nx = A()\r\nassert x.attr is None\r\nx.attr = 5\r\nassert x.attr == 5\r\ndel x.attr\r\nassert x.attr is None\r\n```\r\n\r\nWhat if replace `_Py_T_OBJECT` with `Py_T_OBJECT_EX`? It turns out that you can replace it in 105 sites but 31 sites should keep `_Py_T_OBJECT` to make existing tests pass. This is not a very reliable result because the tests may not cover all cases. On the other hand, some tests are too picky and check the attributes of a newly created uninitialized object, even if they are normally initialized.\r\n\r\nIn any case, we can take these results and replace `_Py_T_OBJECT` with `Py_T_OBJECT_EX` on case by case basis.\r\n\r\n@vstinner @encukou\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-107254\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/serhiy-storchaka","@type":"Person","name":"serhiy-storchaka"},"datePublished":"2023-07-25T16:35:37.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":8},"url":"https://github.com/107253/cpython/issues/107253"}