Django ComputedField

ComputedField.py

class ComputedField(TextField):
    def __init__(self, *args, **kwargs):
        kwargs['editable'] = False
        kwargs['blank'] = True
        if 'computation' not in kwargs:
            raise AttributeError("ComputedField *REQUIRES* computation parameter")
        self.computation = kwargs.pop('computation')
        if not callable(self.computation):
            raise AttributeError("ComputedField computation parameter *MUST* be callable")
        super(ComputedField, self).__init__(*args, **kwargs)

    def pre_save(self, model_instance, add):
        value = ''
        if not add:
            value = self.computation(model_instance)
        setattr(model_instance, self.attname, value)
        return value

ComputedFieldUsage.py

class Entry(models.Model):
    title = models.CharField(max_length = 200, verbose_name = u'Заголовок')
    pubdate = models.DateField(verbose_name = u'Дата публикации', auto_now = True)
    text = models.TextField(verbose_name = u'Текст')
    shortlink = ComputedField(computation = lambda w: googgl.shorten_url('<domainname>' + w.get_absolute_url()))