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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Misleading \"Not an instance or subtype\" error when assigning to property of frozen, slotted dataclass","articleBody":"# Bug report\n\n### Bug description:\n\nWhen a dataclass is frozen, attempting to assign to a property (with a setter or without) fails as expected:\n\n```python\nfrom dataclasses import dataclass\n\n@dataclass(frozen=True)\nclass Example:\n\ta: int\n\n\t@property\n\tdef prop(self):\n\t\treturn None\n\nexample = Example(10)\nexample.prop = 0\n```\n```\n  File \"test_dataclass.py\", line 12, in \u003cmodule\u003e\n    example.prop = 0\n    ^^^^^^^^^^^^\n  File \"\u003cstring\u003e\", line 15, in __setattr__\ndataclasses.FrozenInstanceError: cannot assign to field 'prop'\n```\nAnd when the class is slotted, if the property has no setter, it also fails the way one would expect:\n```python\nfrom dataclasses import dataclass\n\n@dataclass(slots=True)\nclass Example:\n\ta: int\n\n\t@property\n\tdef prop(self):\n\t\treturn None\n\nexample = Example(10)\nexample.prop = 0\n```\n```\nTraceback (most recent call last):\n  File \"test_dataclass.py\", line 12, in \u003cmodule\u003e\n    example.prop = 0\n    ^^^^^^^^^^^^\nAttributeError: property 'prop' of 'Example' object has no setter\n```\n(In this case, of course, adding a setter makes it work fine.)\n\nWhat's unexpected (and confusing) is when the class is both frozen *and* slotted:\n```python\nfrom dataclasses import dataclass\n\n@dataclass(frozen=True, slots=True)\nclass Example:\n\ta: int\n\n\t@property\n\tdef prop(self):\n\t\treturn None\n\nexample = Example(10)\nexample.prop = 0\n```\n```\nTraceback (most recent call last):\n  File \"test_dataclass.py\", line 12, in \u003cmodule\u003e\n    example.prop = 0\n    ^^^^^^^^^^^^\n  File \"\u003cstring\u003e\", line 16, in __setattr__\nTypeError: super(type, obj): obj (instance of Example) is not an instance or subtype of type (Example).\n```\nOne would, of course, expect an instance of `Example` to in fact be an instance of `Example`, but maybe this is somehow a collision between the initial vs. dataclass-ified class definitions?\n\n### CPython versions tested on:\n\n3.13\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-143971\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/HalfWhitt","@type":"Person","name":"HalfWhitt"},"datePublished":"2026-01-17T15:51:14.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/143969/cpython/issues/143969"}