I guess simply using inspect.signature won't be enough, as it must be called with annotation_format=annotationlib.Format.FORWARDREF (which is only available starting in Python 3.14). I'd suggest something around #20024. As inspect.signature is available since Python 3.3 I do not think a fallback to getfullargspec is required. Maybe

from django.utils.version import PY314
if PY314:
    import annotationlib

# ...

if PY314:
    sig = inspect.signature(func, annotation_format=annotationlib.Format.FORWARDREF)
else:
    sig = inspect.signature(func)
alen = len(sig.parameters)

non_default_params = [
    p for p in sig.parameters.values() if p.default is p.empty
]
dlen = alen - len(non_default_params)

could work. A test case could probably be

from typing import TYPE_CHECKING
from django.utils.version import PY314

if TYPE_CHECKING:
    from django.utils.safestring import SafeText 


class SomeTestCase(UnitTest):
    @unittest.skipUnless(PY314, "Deferred annotations are Python 3.14+ only")
    def test_register_filter_deferred_annotations(self):
        register = template.Library()

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

        # Inspection during register fails with deferred annotations with python 3.14+
        # TODO: Maybe check if filter can be used

Thanks for your suggestions, i implemented and tested again these changes with other python versions, it seems ok for now, but if i need to do other improvements, im here to help ☺️

Thanks for the updates @AugustoPontes1. Re-reading the issue, I'm realizing we have similar issues cropping up in several more places (you can grep for where else we're using getfullargspec().

I think we're going to need to factor out a helper for this, and just unit test that.

But an upstream patch would be simpler. I'm going to wait a few days to see if any responds on python/cpython#141560.

Thanks for the updates @AugustoPontes1. Re-reading the issue, I'm realizing we have similar issues cropping up in several more places (you can grep for where else we're using getfullargspec().

I think we're going to need to factor out a helper for this, and just unit test that.

But an upstream patch would be simpler. I'm going to wait a few days to see if any responds on python/cpython#141560.

No problem, i also saw some PRs opened on the cpython repo that are talking about this problem, but they discussed this problem a long time ago, and from what i saw, they didnt get a solution for this, and you remembered well that there are other modules on django that uses the getfullargspec(), and definetely we will need a better solution for this and other problems