Authenticate with Email Address

import ldap
from django.contrib.auth.models import User
from django.core.validators import email_re

AUTH_LDAP_SERVER = 'xxx.xxx.xxx.xxx'

class Authenticate:
    def authenticate(self, username=None, password=None):
        try:
            user = User.objects.get(email=username)
            if user.check_password(password):
                return user
            else:
                try:
                    l = ldap.open(AUTH_LDAP_SERVER)
                except ldap.LDAPError, e:
                    return None
                
                try:
                    # Attempt to bind to the user's DN
                    l.simple_bind_s(username, password)
                    
                    try:
                        user = User.objects.get(email__exact=username)
                    except:
                        return None
                    
                    # Success.
                    return user
                except ldap.INVALID_CREDENTIALS:
                    return None
        except User.DoesNotExist:
                return None
            
    def get_user(self, user_id):
        try:
            return User.objects.get(pk=user_id)
        except User.DoesNotExist:
            return None