mark_safe() doesn't work as a decorator for template filters and tags

[Torsten Bronger]

With Django 1.11, I had the following pattern frequently in my code:

@register.filter
@mark_safe
def myfilter(value):
    ...

Smilarly for tags. This has worked in Django 1.11, however, with Django 2.0, this leads to the TemplateError saying that "myfilter requires 0 arguments, 1 provided". For tags, it results in an IndexError because the "params" list when processing the tag is empty.

If I make @mark_safe the outmost (i.e. first) decorator, no exception occurs – but the output is escaped HTML instead of passed-through HTML.

[Torsten Bronger]

Taceback when using @mark_safe with a template filter function.

[Tim Graham]

Bisected to 620e9dd31a2146d70de740f96a8cb9a6db054fc7.

[Ryan Rubin]

Replying to oliver:
I'm happy to work on this if you need to relinquish it.

[Ryan Rubin]

I went ahead and took a look, since it was 10 days past claimed. I was unable to reproduce for any tags, but I did for filters. As of the bisected commit, inspect.getfullargspec is being passed the function, but if that function is decorated, inspect.getfullargspec does not read __wrapped__ attributes as of Python 3.4: ​https://docs.python.org/3/library/inspect.html#inspect.getfullargspec

The function is checked for a _decorated_function attribute, but that's not always present. inspect.unwrap will get the original function for the call to inspect.getfullargspec, though. I've opened a PR here: ​https://github.com/django/django/pull/9979

[Tim Graham <timograham@…>]

In a8d12bc2:

Fixed #29400 -- Fixed crash in custom template filters that use decorated functions.

Regression in 620e9dd31a2146d70de740f96a8cb9a6db054fc7.

[Tim Graham <timograham@…>]

In 6b91152:

[2.1.x] Fixed #29400 -- Fixed crash in custom template filters that use decorated functions.

Regression in 620e9dd31a2146d70de740f96a8cb9a6db054fc7.

Backport of a8d12bc28069d56e0306770ab9c73738293663f7 from master

[Tim Graham <timograham@…>]

In c2a1af88:

[2.0.x] Fixed #29400 -- Fixed crash in custom template filters that use decorated functions.

Regression in 620e9dd31a2146d70de740f96a8cb9a6db054fc7.

Backport of a8d12bc28069d56e0306770ab9c73738293663f7 from master