is_simple_callable raises NameError on Python 3.14 when method has TYPE_CHECKING-guarded annotations

[simonpercivall]

is_simple_callable() calls inspect.signature(obj) which, under Python 3.14's
PEP 649 (lazy annotation evaluation), eagerly evaluates annotation thunks. This
raises NameError when the annotated method uses types imported under
if TYPE_CHECKING:.

The function only inspects parameter kinds and defaults — it never uses annotations.
The fix is to pass annotation_format=annotationlib.Format.FORWARDREF to
inspect.signature(), which replaces unresolvable names with ForwardRef objects
instead of raising.

Steps to reproduce

# models.py
from typing import TYPE_CHECKING
from django.db import models

if TYPE_CHECKING:
    from myapp.other import SomeType

class MyModel(models.Model):
    name = models.CharField(max_length=100)

    def get_display_name(self) -> "SomeType":
        return self.name

# serializers.py
from rest_framework import serializers

class MyModelSerializer(serializers.ModelSerializer):
    display_name = serializers.CharField(source="get_display_name")

    class Meta:
        model = MyModel
        fields = ["name", "display_name"]

On Python <= 3.13 (or with from __future__ import annotations), this works fine.
On Python 3.14, serializing a MyModel instance raises:

NameError: name 'SomeType' is not defined

The traceback goes through:

• rest_framework.fields.get_attribute (line 107) or
  rest_framework.relations (line 178)
• is_simple_callable (line 82)
• inspect.signature(obj) evaluates the annotation thunk
• annotationlib._get_dunder_annotations triggers NameError

Expected behavior

is_simple_callable should not evaluate annotations, since it only checks whether
parameters have defaults.

Suggested fix

import sys

def is_simple_callable(obj):
    ...
    kwargs = {}
    if sys.version_info >= (3, 14):
        import annotationlib
        kwargs["annotation_format"] = annotationlib.Format.FORWARDREF

    sig = inspect.signature(obj, **kwargs)
    ...

Or, since annotationlib is stdlib in 3.14+:

try:
    import annotationlib
    _SIGNATURE_KWARGS = {"annotation_format": annotationlib.Format.FORWARDREF}
except ImportError:
    _SIGNATURE_KWARGS = {}

def is_simple_callable(obj):
    ...
    sig = inspect.signature(obj, **_SIGNATURE_KWARGS)
    ...

Environment

• Python: 3.14.4
• Django REST framework: 3.17.1 (also affects 3.16.1)
• Django: 5.2+

[browniebroke]

Thanks for the report and detailed explanation. The suggested fix looks good to me.

I have a preference for the if sys.version_info >= (3, 14) version as it makes it clearer when we can remove the condition (which can even be automated).

[simonpercivall]

The suggested fix is a bit light on the comment side, not really explaining why this is needed for Python 3.14+. But I guess that's more a project standard kind of thing (does it go in code with the if, or does it go in the commit message, and so on).

[browniebroke]

If we can write a failing test first which passes when the fix is added that would help with that. But sure, a short comment would be welcome. It could give a one sentence explanation and link to this discussion for more context.