Allow immutable extra_context on TemplateViews

[samueljsb]

Prior to this change, the extra_context attribute of a template view was annotated as a dict (or None). This means that if a value is provided, it must be a mutable object. Since this is an attribute on the class, this creates mutable shared state, which is a source of bugs. It was not possible to use an immutable mapping (MappingProxyType or the 3rd-party frozendict`) without getting type-checking errors.

However, there is no need for this attribute to be mutable. This change allows any mapping type, which allows authors to write template views with immutable extra_context attributes.

Refs #1992

[sobolevn]

This case is explicitly documented to be dict: https://docs.djangoproject.com/en/5.0/ref/class-based-views/mixins-simple/#django.views.generic.base.ContextMixin.extra_context

[samueljsb]

I don't think that documentation is intended to be an exhaustive definition of the types that can be passed in, though. For example, it doesn't mention that the extra context can be None.

Moreover, the way extra_context is being used does not require mutability (it is being passed to dict.update, which will accept any mapping), so it is acceptable to set an immutable mapping on that attribute. Django will never mutate the extra_context on a view class because doing so would leak state between requests.

Is that sufficient for django-stubs to annotate it to allow immutable objects here?

[adamchainz]

IMO yes, let's do it.

[sobolevn]

Thank you all for the feedback! 👍

[samueljsb]

It has just been pointed out to me that the commit message is incorrect - it should have been "allow immutable extra_context". I've fixed the PR title, but whoever writes the changelog will want to make sure they get that the right way around. Sorry!

[sobolevn]

No worries, it is fine :)