Add __length_hint__ methods to itertools helpers (original) (raw)

Following-up on #145333, I would like to add more __length_hint__ methods to itertools helper iterators. Not all of them can have such a method (those that are not finite, like count or unknown like takewhile) but I think we could at least add them for starmap, batched, and possibly others like product, pairwise etc that will require some more attention.

I plan to work on this in the next few days.