[Django] #35578: Customizing `username_validator` in custom User model extending `django.contrib.auth.models.AbstractUser` doesn't work

[Django]

#35578: Customizing `username_validator` in custom User model extending
`django.contrib.auth.models.AbstractUser` doesn't work
------------------------------+-----------------------------------------
     Reporter:  Oscar Rovira  |                     Type:  Uncategorized
       Status:  new           |                Component:  contrib.auth
      Version:  5.0           |                 Severity:  Normal
     Keywords:                |             Triage Stage:  Unreviewed
    Has patch:  0             |      Needs documentation:  0
  Needs tests:  0             |  Patch needs improvement:  0
Easy pickings:  0             |                    UI/UX:  0
------------------------------+-----------------------------------------
 Hi there!
 I'm sorry if that is the expected behaviour of what I'm going to report,
 but looking at the implementation, it should work as I expected.
 I have my own authentication `User` model, extending from `AbstracUser`.
 Looking at the code of `AbstractUser`, one may think that defining the
 class prop `username_validator`, the validation of the username at all the
 levels (createsuperuser command, Admin create user form,
 User.objects.create_user, ...) should use the new validator provided in
 the customized class. It is not working like that. In order to make it
 work, one has to re-define also the `username` field in the customized
 model.

 So, I have the following:
 {{{
 @deconstructible
 class CustomUnicodeUsernameValidator(validators.RegexValidator):
     """Overrides the default username validator from Django Abstract
     User to avoid usernames with characters '@', '+' or '.'
     """
     regex = r"^[\w-]+\Z"
     message = _(
         "Enter a valid username. This value may contain only letters, "
         "numbers, and -/_ characters."
     )
     flags = 0

 ...
 class User(AbstractUser):
     """Main app user, extending default Django users."""

     # overriding the one defined in AbstractUser
     username_validator = CustomUnicodeUsernameValidator() # type: ignore

     # Other custom fields
     ...
 }}}

 and it does not work, the custom username_validator is not being used.

 However, if I redefine the `username` field, including the
 `validators=[username_validator]` attribute, it works as expected.
 As I said, maybe it is the expected behaviour, and it's just a lack of
 documentation.

 Thanks!
-- 
Ticket URL: <https://code.djangoproject.com/ticket/35578>
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