isUnique validator for newforms

from django.db.models import Model
from django.forms import ValidationError
from django.utils.translation import ugettext as _
from django.utils.text import capfirst

def isUnique( instance, field, data ):
    "Validates that a value is unique for a field."
    
    if not isinstance( instance, Model ):
        raise TypeError, u'The instance passed to isUnique is not a subclass of django.db.models.Model'
    
    model = instance.__class__
    
    try:
        matching_obj = model.objects.get( **{ field: data } )
    except model.DoesNotExist:
        return data
    
    if instance and instance.pk == matching_obj.pk:
        return data
    
    raise ValidationError, _( u'%(optname)s with this %(fieldname)s already exists.' ) % {'optname': capfirst( model._meta.verbose_name ), 'fieldname': model._meta.get_field( field, many_to_many = False ).verbose_name }