For python 3.10+ class property may not be accessible by Django's template system

[Fabian Braun]

Prior to python 3.10 class properties were always available through the template system. If you had a class

class MyClass:
    in_template = True

you could access the class property in the template through {{ context_value.in_template }}.

The template system first checks if the class is subscriptable (i.e. tries context_valuein_template), will fail with that and then will get the in_template property.

As of Python 3.10 some classes actually are subscriptable and trying to get the item will not fail: Typing shortcuts introduced syntax like list[int]. This effectively hides class properties 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": "{{ dict.klass.in_template }}"})
    def test_access_class_property(self):
        class MyClass(list):
            in_template = True

            def __init__(self, non_trivial_init):
                # This prevents the template system from turning the class into an instance
                return super().__init__()

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

I'd be happy to propose a fix.