code: def populate_username(self, request, user):
          
          from .utils import user_username, user_email, user_field
          first_name = user_field(user, 'first_name')
          last_name = user_field(user, 'last_name')
          email = user_email(user)
          username = user_username(user)
          if app_settings.USER_MODEL_USERNAME_FIELD:
              user_username(
                  user,
                  username or self.generate_unique_username([
                      first_name,
                      last_name,
                      email,
                      username,
                      'user']))
docs: Fills in a valid username, if required and missing.  If the
          username is already present it is assumed to be valid
          (unique).