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

you could access the class property in the template through (if it was returned by a callable) {{ get_my_class.in_template }}.

The template system first executes the callable get_my_class which we assume returns MyClass. 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 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

        def get_my_class():
            return MyClass

        # Pass the callable to return the class, or it would be resolved by the template
        # engine
        output = self.engine.render_to_string(
            "basic-syntax19b",
            {"klass": get_my_class}
        )
        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