Custom authentication backend. Django

While Bartek's answer is the correct one I'll just provide an example of another way to solve the problem by inheriting the ModelBackend.

from django.contrib.auth.backends import ModelBackend

class EmailAuthBackend(ModelBackend):
    def authenticate(self, username=None, password=None, **kwargs):
        try:
            user = User.objects.get(email=username)
            if user.check_password(password):
                return user
        except ObjectDoesNotExist:
            # Run the default password hasher once to reduce the timing
            # difference between an existing and a non-existing user (#20760).
            User().set_password(password)

The get_user is already implemented by the ModelBackend and you get the permission methods along with it.

For each custom backend in Django, you need to specify the get_user function. See the documentation. The get_user implementation can simply use the existing User table, like you are:

def get_user(self, user_id):
   try:
      return User.objects.get(pk=user_id)
   except User.DoesNotExist:
      return None

The reason this is required is for situations where you'd need to fetch the User via its primary key from a different source.