Allow using forms of contrib.auth without installing contrib.auth

[Sagar Chalise]

It seems the forms available in contrib.auth app cannot be used without using contrib.auth in settings.INSTALLED_APPS.
May be break some of reusable feature of forms into base_forms.py just like AbstractBaseUser model.

[Tim Graham]

Which forms are you trying to use and what error do you get?

[Sagar Chalise]

Basically, I am using custom user model which avoids both groups and permissions. I could easily use Authentication as well as Password related forms but seems the forms.py directly imports User from contrib.auth.models ​https://github.com/django/django/blob/master/django/contrib/auth/forms.py#L10. Which seems to enforce me to install contrib.auth package.

[Tim Graham]

I wonder if moving that import inside the form class where it's used would be feasible and solve the issue.

[Tim Graham <timograham@…>]

In 3333d93:

Fixed #28757 -- Allowed using contrib.auth forms without installing contrib.auth.

Also fixed #28608 -- Allowed UserCreationForm and UserChangeForm to
work with custom user models.

Thanks Sagar Chalise and Rômulo Collopy for reports, and Tim Graham
and Tim Martin for reviews.

[Tim Graham]

After this commit, there are some errors when running $ ./tests/runtests.py auth_tests.test_management (but adding --reverse fixes it). shangdahao, can you debug that?

[Tim Graham]

I tried to solve the issue by putting the reload of django.contrib.auth.forms in django.test.signals.user_model_swapped() rather than limiting it to the Django tests. I couldn't get it to work.

The issue is solved by adding import django.contrib.auth.forms to the top of auth_tests/test_management.py but I don't know why that works.

[Sławek Ehlert]

Tim, I think the commit from this ticket doesn't properly support all the cases of custom user models (i.e. it assumes that the username field should be a subclass of CharField). I've opened #29449 for that. Also there's something weird going on with running the tests (details are in the ticket).

[Tim Graham <timograham@…>]

In f3fa86a:

Fixed #29449 -- Reverted "Fixed #28757 -- Allowed using contrib.auth forms without installing contrib.auth."

This reverts commit 3333d935d2914cd80cf31f4803821ad5c0e2a51d due to
a crash if USERNAME_FIELD isn't a CharField.

[Tim Graham <timograham@…>]

In 78f502c:

[2.1.x] Fixed #29449 -- Reverted "Fixed #28757 -- Allowed using contrib.auth forms without installing contrib.auth."

This reverts commit 3333d935d2914cd80cf31f4803821ad5c0e2a51d due to
a crash if USERNAME_FIELD isn't a CharField.

Backport of f3fa86a89b3b85242f49b2b9acf58b5ea35acc1f from master

[Tim Graham]

I reverted the original patch due to the regression described in #29449. Unit tests for those cases are provided in ​PR 10020 which can be included in the patch for this ticket.