Investigate possible misuse of Context

[Aymeric Augustin]

This was reported in the comments of #17229, but I'm moving it to its own ticket for clarity.

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Changeset r17894 produced several errors in the Django test suite. In fact, it reveals issues where Context instances are initialized with a Context argument instead of a simple dict.
Examples:

• In browser:django/trunk/django/contrib/auth/tests/urls.py, several render_to_response calls have the Context instance as second argument, which seems to be plain bugs (uncaught until now)
• In browser:django/trunk/django/contrib/admin/templatetags/admin_modify.py, the prepopulated_fields_js inclusion tag returns a Context instance instead of a dict.

The question is now whether Context initialization should accept other Context instance as the __init__ dict_ parameter (and then this changeset should be fixed) or if we should enforce dict_ as a dict and fix places in Django code when it is not the case. I'd be in favour of the latter, but this might need some more investigation.

[Claude Paroz]

After re-reading the docs about Context, I think things are clear: Context takes a dict as optional init parameter. So for me, passing a Context to another Context init is a mistake and should be fixed (see my patch on #18103).

[Aymeric Augustin]

I think we need a deprecation path, but I'm in favor of this change.

[Claude Paroz]

Do not accept context instance as second render_to_response parameter

[Claude Paroz]

Properly construct context when inclusion tag returns a context

[Claude Paroz]

I have separated in two distinct patches the problematic uses of Context initialization from a context instance in Django code.

At this point and considering the correction you committed in r17896, it's still possible to intialize a Context from another Context, even with these patches.

[Aymeric Augustin]

733178830072caeca3c054a220808b4c557faec4 changed render_to_response so it wouldn't rewrap a Context inadvertently passed in the dictionary argument in another Context. That's the opposite of the route taken by first patch above and I'm convinced it's a better approach given how widespread render_to_response is and how easy forgetting context_instance= is.

The second patch still warrants consideration. I had a look and I don't think the code inside if isinstance(_dict, BaseContext) is correct: it adds values such as autoescape and current_app to the context when they're intended to be attributes of the context object.

[Mariusz Felisiak]

As far as I'm aware the second part was fixed by e53495ba3352c2c0fdb6178f2b333c30cb6b5d46.