Fixed #17450 -- Added missing import to auth models module forgotten in r17253. Thanks, aalbrecht.

from django.core.mail import send_mail

from django.db import models

from django.db.models.manager import EmptyManager

from django.utils.crypto import get_random_string

from django.utils.encoding import smart_str

from django.utils.translation import ugettext_lazy as _

from django.utils import timezone

    def get_by_natural_key(self, codename, app_label, model):

        return self.get(

            codename=codename,

            content_type=ContentType.objects.get_by_natural_key(app_label, model)

            content_type=ContentType.objects.get_by_natural_key(app_label, model),

        )

class Permission(models.Model):