That doesn't look faster -- 60ms? Nor clearer, at least to me :) Additionally, you might have a bug in the code: groupby() only groups consecutive keys, so you

That doesn't look faster -- 60ms? Nor clearer, at least to me :) Additionally, you might have a bug in the code: groupby() only groups consecutive keys, so your code will fail if the dataset is not already sorted by product_id. I guess that's the lesson here -- use pandas to make sure you get a sane, fast, tested implementation of some common use-cases. That is, if you can remember its API, when it makes copies vs views under the hood, and all that gnarly stuff :-)