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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Frozen dataclasses with slots cannot override __getstate__ or __setstate__","articleBody":"You are supposed to be able to override how `pickle` pickles an object with `__getstate__` and `__setstate__`. However, these methods [are ignored](https://stackoverflow.com/questions/76143398/pickle-ignores-getstate-on-frozen-dataclasses-with-slots) in a dataclass when specifying both `frozen=True` and `slots=True`.\r\n\r\nSee this example:\r\n\r\n```python\r\nimport pickle\r\nfrom dataclasses import dataclass\r\n\r\n\r\n@dataclass(frozen=True, slots=True)\r\nclass Foo:\r\n    bar: int\r\n\r\n    def __getstate__(self):\r\n        print(\"getstate\")\r\n        return {\"bar\": self.bar}\r\n\r\n    def __setstate__(self, state):\r\n        print(\"setstate\")\r\n        object.__setattr__(self, \"bar\", state[\"bar\"])\r\n\r\nb = pickle.dumps(Foo(1))\r\nfoo = pickle.loads(b)\r\n```\r\n\r\nThe expected \"getstate\" and \"setstate\" lines are never printed because the supplied methods are never called. If either `frozen` or `slots` is removed, the expected lines are printed.\r\n\r\nFrom [the source code](https://github.com/python/cpython/blob/main/Lib/dataclasses.py#L1228-L1231), it is pretty clear why this is happening. If `frozen` and `slots` are both `True`, then special versions of `__getstate__` and `__setstate__` are unconditionally attached to the dataclass. The dataclass decorator should probably treat this way that it does other methods—only add a method if it is not already present.\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-104041\n* gh-104044\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/drhagen","@type":"Person","name":"drhagen"},"datePublished":"2023-05-01T13:04:51.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/104035/cpython/issues/104035"}