René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Cannot specify `weakref_slot=True` twice in dataclass MRO","articleBody":"## Summary\r\n\r\nIf a child dataclass of a parent dataclass who specified `weakref_slot`, also decides to set `weakref_slot` to True, Python will raise a TypeError as seen below:\r\n```\r\nTypeError: __weakref__ slot disallowed: either we already got one, or __itemsize__ != 0\r\n```\r\n\r\n## Description\r\n\r\nI am porting dataclasses' slots and weakref slots kwargs into a decorator I can use. While doing so, I decided to change the default of the `weakref_slot` because I would like all of my classes to be weakref:able. This is when I ran into the issue which can be reproduced by the code below:\r\n```python\r\nfrom dataclasses import dataclass\r\n\r\n\r\n@dataclass(slots=True, weakref_slot=True)\r\nclass A:\r\n    field: str\r\n\r\n\r\n@dataclass(slots=True, weakref_slot=True)\r\nclass B(A):\r\n    ...\r\n```\r\n```\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 2, in \u003cmodule\u003e\r\n  File \"C:\\Users\\%username%\\Projects\\testing\\copyclasses.py\", line 1207, in wrap\r\n    return _process_class(cls, init, repr, eq, order, unsafe_hash,\r\n  File \"C:\\Users\\%username%\\Projects\\testing\\copyclasses.py\", line 1108, in _process_class\r\n    cls = _add_slots(cls, frozen, weakref_slot)\r\n  File \"C:\\Users\\%username%\\Projects\\testing\\copyclasses.py\", line 1176, in _add_slots    \r\n    cls = type(cls)(cls.__name__, cls.__bases__, cls_dict)\r\nTypeError: __weakref__ slot disallowed: either we already got one, or __itemsize__ != 0\r\n```\r\n\r\n### Environment\r\n\r\nThis was tested by creating a file named `copyclasses.py` with the current contents of `Lib/dataclasses.py` ran using a 3.10 CPython interpreter. Note that I've restored the naming of the module in the codeblock above assuming those who will be running it will have a built 3.11 CPython interpreter.\r\n\r\n---\r\n\r\nFootnotes: I will mention @ericvsmith as the code owner of the dataclasses file.","author":{"url":"https://github.com/Bluenix2","@type":"Person","name":"Bluenix2"},"datePublished":"2022-06-05T21:42:22.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":5},"url":"https://github.com/93521/cpython/issues/93521"}