gh-132261: Store annotations at hidden internal keys in the class dict #132345
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carljm
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Apr 11, 2025
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This looks pretty good to me! I expected it to be a lot gnarlier. It really just removes the ugly special handling of class annotations, everything it adds looks predictable and reasonably straightforward.
Thanks for your work on this!
Misc/NEWS.d/next/Core_and_Builtins/2025-04-09-21-51-37.gh-issue-132261.gL8thm.rst
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…e-132261.gL8thm.rst Co-authored-by: Carl Meyer <carl@oddbird.net>
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Following python/cpython#132345, type annotations result in the annotate function being stored at `__annotate_func`. This caused test failures in klein along the lines of: ``` zope.interface.exceptions.BrokenMethodImplementation: The object FrozenHTTPHeaders(rawHeaders=()) has failed to implement interface klein._imessage.IHTTPHeaders: The contract of klein._imessage.IHTTPHeaders.__annotate_func__(format) is violated because 'FrozenHTTPHeaders.__annotate__()' doesn't allow enough arguments. ``` Ignore this new internal name.
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Following python/cpython#132345, type annotations result in the annotate function being stored at `__annotate_func`. This caused test failures in klein along the lines of: ``` zope.interface.exceptions.BrokenMethodImplementation: The object FrozenHTTPHeaders(rawHeaders=()) has failed to implement interface klein._imessage.IHTTPHeaders: The contract of klein._imessage.IHTTPHeaders.__annotate_func__(format) is violated because 'FrozenHTTPHeaders.__annotate__()' doesn't allow enough arguments. ``` Ignore this new internal name.
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This fixes #132261 and provides a more elegant solution for https://peps.python.org/pep-0749/#annotations-and-metaclasses. The trouble is that we store things at the
__annotate__and__annotations__keys in the class dictionary and they interfere with the descriptors on type: the solution is to not store our internal objects at those keys. Instead, we now store the annotate function at__annotate_func__and the annotations dict (once evaluated) at__annotations_cache__. These names remain undocumented implementation details.Downsides:
typing.pyneeded some updates to deal with this.__annotations__. In 3.13, this always worked: if a class has annotations there's an__annotations__key in the class dict. On current main, it sometimes works (only if somebody has accessed the.__annotations__attribute). With this PR, it never works (unless the__annotations__key was explicitly added). Breaking this pattern consistently seems better than breaking it nondeterministically though.__new__. I madeannotationlib.get_annotate_function()handle this.cls.__annotate_func__).get_annotationson instances gets an unexpected error #132261📚 Documentation preview 📚: https://cpython-previews--132345.org.readthedocs.build/