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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"__class_getitem__ Unexpectedly Falls Back to the Metaclass","articleBody":"# Bug report\r\n\r\n### Bug description:\r\n\r\nA major point of `__class_getitem__` ([PEP 560](https://peps.python.org/pep-0560/)) is to avoid metaclasses.  This implies that the metaclass should never be involved in the mechanism.  However, currently (and since the feature landed) we actually do fall back to the metaclass:\r\n\r\n\u003cdetails\u003e\r\n\u003csummary\u003e(example)\u003c/summary\u003e\r\n\r\n```python\r\nclass Spam:\r\n    def __class_getitem__(cls, key):\r\n        return f'spam got {key!r}'\r\n\r\n\r\nclass Meta(type):\r\n    def __class_getitem__(cls, key):\r\n        return f'meta got {key!r}'\r\n\r\n\r\nclass Eggs(metaclass=Meta):\r\n    def __class_getitem__(cls, key):\r\n        return f'eggs got {key!r}'\r\n\r\n\r\nclass Ham(metaclass=Meta):\r\n    pass\r\n\r\nprint(Spam[10])\r\nprint(Eggs[10])\r\nprint(Ham[10])\r\n```\r\n\r\nOutput:\r\n\r\n```\r\nspam got 10\r\neggs got 10\r\nmeta got 10\r\n```\r\n\r\n\u003c/details\u003e\r\n\r\nI was expecting the last one to raise `TypeError: type 'object' is not subscriptable`, since `Ham` does not implement `__class_getitem__`.  The actual outcome is surprising because the metaclass can already define `__getitem__`.  Falling back to the metaclass `__class_getitem__` doesn't make much sense and can be confusing.  The metaclass `__class_getitem__` should only be used when subscripting the metaclass, not its instances.\r\n\r\nThe PEP doesn't really address the question of a metaclass that defines `__class_getiem__` [^1] (nor does the documentation as far as I noticed).  Overall, it seems like this was simply not noticed nor considered.  It's certainly not an obvious case.  Regardless, I think we should fix it.\r\n\r\n[^1]: The PEP does say `Note that this method is used as a fallback, so if a metaclass defines __getitem__, then that will have the priority.` but that's specifically about falling back to `meta.__getitem__`.\r\n\r\nThe fix would involve skipping the metaclass part.  That would be in the implementation for the subscript syntax (`PyObject_GetItem()` in Objects/abstract.c).  There's a part where it specially handles the case where a class is being subscripted.  In that case it looks up `__class_getitem__` on the class.  (See gh-4732.)  However, currently it uses `PyObject_GetOptionalAttr()`, which involves descriptors and the metaclass (the object's type) and the type's `__mro__`.  Again, the fix is to skip the metaclass part.\r\n\r\nFWIW, `__init_subclass__` is fairly similar, but it doesn't fall back to the metaclass.  Instead, it effectively does `getattr(super(cls), '__init_subclass__')`.  (See `type_new_init_subclass()` in Objects/typeobject.c.)  We should probably do something similar in `PyObject_GetItem()`.\r\n\r\nCC @ilevkivskyi @gvanrossum\r\n\r\n### CPython versions tested on:\r\n\r\nCPython main branch\r\n\r\n### Operating systems tested on:\r\n\r\n_No response_\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-122743\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ericsnowcurrently","@type":"Person","name":"ericsnowcurrently"},"datePublished":"2024-08-03T00:10:24.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/122634/cpython/issues/122634"}