[ufo]

I cried a little bit on the inside when he said that that the iterator should not be implemented using generators, etc. Writing iterators by hand forces one to turn the iteration code inside out and it can be a painful experience if the logic is anything nontrivial.

[andreasvc]

But there's a good motivation here: (x)range objects are supposed to be index-able, while a generator (expression) cannot go back once an element has been consumed.