Allow using test client session handling with custom session apps

[Sergey Kolosov]

This ticket follows up #22634 (which allowed extending database-backed session backends).

While it is now possible to reuse session backend classes, it's not the case for the corresponding parts of the test client.

Namely, in case of having the custom session backend and its corresponding parts (models, middleware, etc) as a separate application, you're unable to use the test client with sessions. The issue is 'django.contrib.sessions' being hardcoded inside of _session and login().

Working that around involves monkey-patching the test client with modified copies of _session and login().

I suggest refactoring test client for better reusability (given this would eliminate session app name being hardcoded).

[Sergey Kolosov]

A possible solution in PR ​https://github.com/django/django/pull/5586

[Raphael Michel]

I think that this is an useful thing to have.

I'm unsure about your PR on first sight, though, as it needs to add a new setting for an edge case and adding settings is generally discouraged. Maybe someone else likes to review it more thouroughly.

[Claude Paroz]

Instead of adding a new setting, I'd rather see an AppConfig attribute (like session_app), and then the _session_app_is_installed could cycle through installed AppConfig's to look for this attribute.

[Sergey Kolosov]

Sounds good, I'll update the PR given the suggested approach.

[Sergey Kolosov]

I'm gonna work on solution suggested in ​https://github.com/django/django/pull/5586#issuecomment-161918171

[Sergey Kolosov]

A new PR: ​https://github.com/django/django/pull/6380

[Sergey Kolosov]

PR updated: ​https://github.com/django/django/pull/6380

[Tim Graham <timograham@…>]

In 21dd98a3:

Fixed #25699 -- Allowed using the test client if 'django.contrib.sessions' isn't in INSTALLED_APPS.