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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Breaking change of `dataclasses.dataclass` comparison semantics in 3.13+","articleBody":"# Bug report\r\n\r\n### Bug description:\r\n\r\n#### Brief Description\r\n\r\nOptimization done in #109870 changed semantic of `dataclass` comparison.\r\n\r\n#### Description\r\n\r\nThe pseudo code below shows meaning of changes have been done in #109870. The semantic differs largely because of a shortcut in `__eq__` implementation of sequence-like containers (see [Objects/object.c][1]). The shortcut essentially does `self[i] is other[i]`. Consequently, method `__eq__` of `self[i]` is not evaluated for identical objects in 3.12 during `dataclasses.dataclass` comparison.\r\n\r\n[1]: https://github.com/python/cpython/blob/v3.13.1/Objects/listobject.c#L3376-L3378\r\n\r\n```python\r\ndef __eq__312(self, other):\r\n    return astuple(self) == astuple(other)\r\n\r\ndef __eq__313(self, other):\r\n    for lhs, rhs in zip(astuple(self), astuple(other)):\r\n        if not lhs == rhs:\r\n            return False\r\n    else:\r\n        return True\r\n``` \r\n\r\nAccording [Python docs](https://docs.python.org/3.13/library/dataclasses.html) (citation below), v3.13 introduces breaking change since it does not consider fields as a tuples for dataclass comparison.\r\n\r\n\u003e eq: If true (the default), an [`__eq__()`](https://docs.python.org/3.13/reference/datamodel.html#object.__eq__) method will be generated. This method compares the class as if it were a tuple of its fields, in order. Both instances in the comparison must be of the identical type.\r\n\r\n\r\n#### Test Case\r\n\r\n```python\r\nimport numpy as np\r\nfrom dataclasses import dataclass\r\n\r\n@dataclass\r\nclass A:\r\n    xs: np.ndarray\r\n\r\na = A(np.ones(3))\r\nb = A(a.xs)\r\n\r\nprint(a == b)  # FAIL (3.13); OK (3.12).\r\n# ValueError: Value The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()\r\n```\r\n\r\n\r\n### CPython versions tested on:\r\n\r\n3.13 (3.12)\r\n\r\n### Operating systems tested on:\r\n\r\nLinux","author":{"url":"https://github.com/daskol","@type":"Person","name":"daskol"},"datePublished":"2024-12-27T15:21:05.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":13},"url":"https://github.com/128294/cpython/issues/128294"}