I'd been wondering why _source was in there. I'm a little surprised that Hettinger thinks it's so important to retain it. It's a neat demonstration of how to make a new data type, and I like that it's so straightforward, but I don't buy the argument that slowing down the interpreter is important for didactic reasons.

Edit: I'm talking about namedtuple, the implementation of which is slow because namedtuple() has to construct _source for every new namedtuple type. Which was clear from the article but not from my comment.