Don't use invalid backend from the Session to load the user in contrib.auth.get_user

[Tamas Szabo]

django.contrib.auth saves the backend_path used to authenticate the user in the session.
This works great but when you remove the authorization backend you get an ImproperlyConfigured error.
The error references the old authorization backend so your first instinct is that the problem must be in your settings file.

I've been bitten by this a few times now, sometimes I remember to delete the Sessions manually, other times (like today) I think that my old settings.py must be executed and I go through the pain of deleting directories, redeploying and restarting servers.

I would like to suggest adding a check to the get_user call, that checks if the backend_path saved in the session is still in settings.AUTHORIZATION_BACKENDS.
If it isn't don't try to load it.

Patch for django.contrib.auth.init.py and tests included.

Thanks.

[Carl Meyer]

Seems sensible. Patch needs updating for trunk (auth backend tests are now in contrib.auth.tests).

[Tamas Szabo]

Added a new version of the patch that moves the tests to contrib/auth/tests/auth_backends.py.
I've also changed the tests to be more similar to the tests already in auth_backends.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tim Graham]

This looks like it has been fixed: ​https://github.com/django/django/blob/19e6397f82e82438eb3047be0233aa2e555b3a12/django/contrib/auth/__init__.py#L155-L157