In pickle.py, load_appends is currently defined as

    def load_appends(self):
        stack = self.stack
        mark = self.marker()
        list = stack[mark - 1]
        list.extend(stack[mark + 1:])
        del stack[mark:]

However, according to the spec of APPENDS, it should actually be:

    def load_appends(self):
        obj = stack[mark - 1]
        items = stack[mark + 1:]
        if isinstance(obj, list):
            obj.extend(items)
        else:
            for item in items:
                obj.append(item)

This will match with the current behaviour of _pickle.