Get the value of the field

def _get(self, instance, **kw):
        """Get the value of the field
        """
        logger.debug("ATFieldManager::get: instance={} field={}"
                     .format(instance, self.field))

        # check the field permission
        if not self.field.checkPermission("read", instance):
            raise Unauthorized("You are not allowed to read the field {}"
                               .format(self.name))

        # return the field value
        return self.field.get(instance)