username = `abcd28`
user_email = `abcd@gmail.com`
user = User.objects.create_user(username, user_email, user_pass)

email = request.POST['email']
password = request.POST['password']
username = User.objects.get(email=email.lower()).username
user = authenticate(username=username, password=password)
login(request, user)

from django.contrib.auth import get_user_model
from django.contrib.auth.backends import ModelBackend

class EmailBackend(ModelBackend):
    def authenticate(self, username=None, password=None, **kwargs):
        UserModel = get_user_model()
        try:
            user = UserModel.objects.get(email=username)
        except UserModel.DoesNotExist:
            return None
        else:
            if getattr(user, 'is_active', False) and user.check_password(password):
                return user
        return None

AUTHENTICATION_BACKENDS = ['path.to.auth.module.EmailBackend']

class MyUser(AbstractUser):
    USERNAME_FIELD = 'email'
    email = models.EmailField(_('email address'), unique=True) # changes email to unique and blank to false
    REQUIRED_FIELDS = [] # removes email from REQUIRED_FIELDS

from django.contrib.auth.backends import ModelBackend
from django.contrib.auth import get_user_model

class YourBackend(ModelBackend):

  def authenticate(self, username=None, password=None, **kwargs):
    UserModel = get_user_model()
    if username is None:
        username = kwargs.get(UserModel.USERNAME_FIELD)
    try:
        if '@' in username:
            UserModel.USERNAME_FIELD = 'email'
        else:
            UserModel.USERNAME_FIELD = 'username'

        user = UserModel._default_manager.get_by_natural_key(username)
    except UserModel.DoesNotExist:
        UserModel().set_password(password)
    else:
        if user.check_password(password) and self.user_can_authenticate(user):
            return user

AUTHENTICATION_BACKENDS = ['path.to.class.YourBackend']

from django.contrib.auth import get_user_model # gets the user_model django     default or your own custom
from django.contrib.auth.backends import ModelBackend
from django.db.models import Q
class CustomBackend(ModelBackend): #requires to define two functions authenticate and get_user
   def authenticate(self, username=None, password=None, **kwargs):
       UserModel = get_user_model()
       try:
       #below line gives query set,you can change the queryset as per your requirement
          user = UserModel.objects.filter(
            Q(username__iexact=username)|
            Q(email__iexact=username)
        ).distinct()

       except UserModel.DoesNotExist:
          return None

        if user.exists():
           user_obj = user.first() # get the user object from the underlying query set , there will only be one object since username and email should be unique fields in your models.
           if user_obj.check_password(password):
               return user_obj
          return  None
       else:
           return None

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