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annotationlib: `ref.evaluate(format=Format.FORWARDREF)` returns a ForwardRef with a copied `__globals__` that no longer updates

DavidCEllis

Bug report

Bug description:

This can happen internally in get_annotations and means subsequent attempts to evaluate it will fail even if the names have since been defined.

Underlying logic:

from annotationlib import get_annotations, Format

class Demo:
    x: Sequence[undefined]

annos = get_annotations(Demo, format=Format.FORWARDREF)

x_anno = annos['x']

# Try to evaluate the reference, but just give a forwardref if it fails
x_repeat_anno = x_anno.evaluate(format=Format.FORWARDREF)

# The resulting reference no longer shares the globals namespace with the annotate function
print(f"{x_anno.__globals__ is Demo.__annotate__.__globals__ = }")  # True
print(f"{x_repeat_anno.__globals__ is Demo.__annotate__.__globals__ = }")  # False

# Define the previously undefined attributes
from collections.abc import Sequence
undefined = str

# This means evaluation fails in the second case
print(f"{x_anno.evaluate() = }")  # collections.abc.Sequence[str]
print(f"{x_repeat_anno.evaluate() = }")  # NameError

This evaluate call happens internally if get_annotations has to rely on the fallback behaviour for an unexpected exception, such as an AttributeError:

from annotationlib import get_annotations, Format
import typing

class Works:
    a: Sequence[undefined]
    b: unknowable

# Intentionally set up an annotation that will raise AttributeError on evaluation
class Fails:
    a: Sequence[undefined]
    b: typing.doesnotexist

a_works = get_annotations(Works, format=Format.FORWARDREF)['a']
a_fails = get_annotations(Fails, format=Format.FORWARDREF)['a']

# Realise the references
from collections.abc import Sequence
undefined = str

print(f"{a_works.evaluate() = }")  # collections.abc.Sequence[str]
print(f"{a_fails.evaluate() = }")  # NameError

This appears to be caused by the creation of a new globals dict here:

if type_params is not None:
globals = dict(globals)
for param in type_params:
globals[param.__name__] = param

Commenting this out makes these examples succeed, but obviously breaks type parameters.

CPython versions tested on:

CPython main branch, 3.14

Operating systems tested on:

No response

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