For python 3.9+ class property may not be accessible by Django's template system

[Fabian Braun]

Before python 3.9 class properties were always available through the template system. If you had a class

class MyClass:
    in_template = True
    do_not_call_in_templates = True  # prevent instantiation

    @classmethod
    def render_all_objects(cls):
        ...

you could access the class property in the template through (if it was contained in the context) {{ MyClass.in_template }} or {{ MyClass. render_all_objects }}.

The template system first gets the class MyClass (and does not instantiate it) or gets it as a result of a callable get_my_class. Then it checks if the class is subscriptable (i.e. tries MyClass["in_template"]), will fail and then will get the in_template property.

As of python 3.9 some classes actually are subscriptable and trying to get the item will not fail: Typing shortcuts introduced syntax like list[int]. These hide class properties or methods from the template system.

Here's a test (that might go into tests/template_tests/syntax_tests/tests_basic.py) which passes on Python 3.9 and fails on Python 3.10+:

    @setup({"basic-syntax19b": "{{ klass.in_template }}"})
    def test_access_class_property(self):
        class MyClass(list):
            in_template = True
            do_not_call_in_templates = True  # prevent instantiation

        output = self.engine.render_to_string("basic-syntax19b", {"klass": MyClass})
        self.assertEqual(output, "True")

I'd be happy to propose a fix that will not call a classes' __class_getitem__ method.

Thanks to ​Ben Stähli and ​Serhii Tereshchenko for figuring out this issue.

References:

• ​https://github.com/django-cms/django-cms/issues/7948