Document that accessing `__annotations__` through `__dict__` no longer works

[patrick-kidger]

Bug report

Bug description:

The following code crashes on 3.14.0a4 but runs without error on 3.13:

import dataclasses

def my_dataclass(cls):
    try:
        annotations = cls.__dict__["__annotations__"]
    except KeyError:
        cls = dataclasses.dataclass()(cls)
    else:
        new_annotations = dict(annotations)
        for name in annotations.keys():
            if name == "ignore_this_annotation":
                new_annotations.pop(name)
        cls.__annotations__ = new_annotations
        cls = dataclasses.dataclass()(cls)
        cls.__annotations__ = annotations
    return cls

@my_dataclass
class X:
    x: int = 3
    ignore_this_annotation: int

with:

TypeError: non-default argument 'ignore_this_annotation' follows default argument 'x'

Python installed via uv venv --python=3.14.

x-ref patrick-kidger/optimistix#168 patrick-kidger/equinox#1096)

CPython versions tested on:

3.14

Operating systems tested on:

macOS

[picnixz]

Does it reproduce for 3.14.0rc3? (I believe that you meant an RC version as your Python is coming from uv which IIRC usea rc2 at least).

Otherwise, cc @JelleZijlstra @ericvsmith

[JelleZijlstra]

Reading annotations from the __dict__ is no longer supported as a result of the implementation of PEP-649. Use annotationlib.get_annotations instead.

[patrick-kidger]

Thank you both for your response!

Does it reproduce for 3.14.0rc3? (I believe that you meant an RC version as your Python is coming from uv which IIRC usea rc2 at least).

I'm just repeating what uv reports 😁

❯ uv run python
Python 3.14.0a4 (main, Jan 14 2025, 23:28:13) [Clang 19.1.6 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

Reading annotations from the __dict__ is no longer supported as a result of the implementation of PEP-649. Use annotationlib.get_annotations instead.

IIUC, all instances of cls.__dict__['__annotations__'] should now be:

import sys

if sys.version_info >= (3, 14):
    import annotationlib
    x = annotationlib.get_annotations(cls)
else:
    x = cls.__dict__['__annotations__']

?

It seems that the meaning of existing code is being silently changed. (As is the case here.) At minimum could it be arranged for cls.__dict__['__annotations__'] to emit an error of some kind? For example, directing them to this issue or suggesting the above fix?

[JelleZijlstra]

You can use inspect.get_annotations on most versions (since 3.10), and typing_extensions.get_annotations on all.

At minimum could it be arranged for cls.dict['annotations'] to emit an error of some kind? For example, directing them to this issue or suggesting the above fix?

No, there's no plausible way we could emit a warning on dict access to an arbitrary key. However, there is a Ruff rule for this: https://docs.astral.sh/ruff/rules/access-annotations-from-class-dict/

[picnixz]

Considering this is the expected behaviour I am going to close this one as "not an issue"

[JelleZijlstra]

I think we should document better that raw __dict__ access no longer works reliably.