Add an extra_context attribute to class-based generic views that use ContextMixin (e.g. TemplateView)

[Jeremy]

It used to be possible to pass extra_context to direct_to_template. Then direct_to_template was replaced with TemplateView.as_view ... but the latter function never got the ability to take an extra_context arg.

As a result there are now many many re-implementations of some variation of the following:

class StaticPageView(TemplateView):
    context = None

    def get_context_data(self, **kwargs):
        context = super(StaticPageView, self).get_context_data(**kwargs)
        context.update(self.context or {})
        return context

You can find them across the web, across StackOverflow, etc. Everyone has to remake this one small class just so that they can pass an extra context to template-only views.

This is counter-productive: the whole point of a framework is to *prevent* everyone from having to re-invent the same code. Adding yet another re-implementation of StaticPageView does nothing to improve the code, it only clutters it up with boilerplate.

Please fix this by allowing as_view (ideally in all generic views) to take an extra_context arg.

[Tim Graham]

This was also suggested in #17697 but the ticket triagers didn't understand the suggestion. The suggestion seems okay to me. I think it would involve modifying ​ContextMixin.get_context_data().

[Bruno Alla]

I'm going to give this a go (unless someone already started).

[Tim Graham]

The ​PR is missing documentation. See the PatchReviewChecklist.

[Bruno Alla]

I assume a good place to document that is here:
​https://docs.djangoproject.com/en/dev/ref/class-based-views/mixins-simple/#contextmixin

As well as adding an entry in the release notes:
​https://docs.djangoproject.com/en/dev/releases/2.0/

And maybe mention it in the TemplateView, under the context section as well?
​https://docs.djangoproject.com/en/dev/ref/class-based-views/base/#templateview

[Tim Graham]

Looks good. ref/class-based-views/flattened-index/ also.

[Tim Graham <timograham@…>]

In 604341c:

Fixed #28331 -- Added ContextMixin.extra_context to allowing passing context in as_view().