Deferred type annotations on template tags lead to TypeError with Python 3.14

[Patrick Rauscher]

Similar to #36696 (which covered mainly Signals) this covers templatetags, where issues may happen when using SafeText:

from typing import TYPE_CHECKING

from django import template
from django.utils.html import escape

if TYPE_CHECKING:
    from django.utils.safestring import SafeText


register = template.Library()

@register.filter("example")
def example_filter(value: str) -> SafeText:
    return escape(value)

When using the templatetag, django/template/base.py line 763 (in django 5.2.8) will call inspect.getfullargspec for the filter, leading to a NameError which gets translated by inspect to a TypeError: unsupported callable. After searching a bit through the code this may also happen with views, as several parts of django use inspect directly for different checks (e.g. for templatetags it is used to check the number of arguments).

Not sure if this is a bug in python or django yet. One solution in django would be to replace direct calls to inspect.signature or inspect.getfullargspec (which uses inspect.signature internally according to python docs, but does not allow to specify an annotation_format.

[Jacob Walls]

Thanks for the report. We should migrate away from ​getfullargspec() anyway:

Note that signature() and Signature Object provide the recommended API for callable introspection, and support additional behaviours (like positional-only arguments) that are sometimes encountered in extension module APIs. This function is retained primarily for use in code that needs to maintain compatibility with the Python 2 inspect module API.

See also this ​deprecation proposal.

[Augusto Pontes]

Hi patrick, after some looking into the problem, it seems that this more a python problem instead of a django problem, because i tested on these versions: 3.12.3, 3.13.2, and the TypeError didnt raised, only at the version 3.14 as you tested, im studying other alternatives to solve this specific problem, depending on my progress, i will open a PR to give a suggestion

[Augusto Pontes]

​https://github.com/django/django/pull/20069

[Simon Charette]

Just renamed the ticket to avoid "deferred annotation" ambiguity with ORM language.

[Augusto Pontes]

Replying to Simon Charette:

Just renamed the ticket to avoid "deferred annotation" ambiguity with ORM language.

Ok, thanks

[Jacob Walls]

​PR

[Jacob Walls]

Natalia approved on github and asked me to merge, marking RFC

[Jacob Walls <jacobtylerwalls@…>]

In 34186e73:

Fixed #36712 -- Evaluated type annotations lazily in template tag registration.

Ideally, this will be reverted when an upstream solution is available for
​https://github.com/python/cpython/issues/141560.

Thanks Patrick Rauscher for the report and Augusto Pontes for the
first iteration and test.

[Jacob Walls <jacobtylerwalls@…>]

In ec732745:

[6.0.x] Fixed #36712 -- Evaluated type annotations lazily in template tag registration.

Ideally, this will be reverted when an upstream solution is available for
​https://github.com/python/cpython/issues/141560.

Thanks Patrick Rauscher for the report and Augusto Pontes for the
first iteration and test.

Backport of 34186e731ca20a2344b1f88fd543a854d6b13a00 from main.

[Jacob Walls <jacobtylerwalls@…>]

In da1dfe64:

[5.2.x] Fixed #36712 -- Evaluated type annotations lazily in template tag registration.

Ideally, this will be reverted when an upstream solution is available for
​https://github.com/python/cpython/issues/141560.

Thanks Patrick Rauscher for the report and Augusto Pontes for the
first iteration and test.

Backport of 34186e731ca20a2344b1f88fd543a854d6b13a00 from main.