gistfile1.py

class _AppendOnlyAttribute(object):
    def __init__(self, name):
        self.name = name

    def __get__(self, instance, owner):
        if instance:
            return getattr(instance._history[0], self.name)
        else:
            return getattr(owner.__history_class__, self.name)

    def __set__(self, instance, value):
        prev_history_record = instance._history[0]
        instance._history.insert(
            0, prev_history_record.fork(**{
                self.name: value,
                'id': instance.next_history_id(),
            }))


def append_only_attr(name, **kwargs):
    retval = _AppendOnlyAttribute(name)
    if 'default_value' in kwargs:
        retval.default_value = kwargs.pop('default_value')
    return retval