See also issue14126.

It makes sense to implement map.__length_hint__() and  zip.__length_hint__(). But note that map() and zip() take several iterables, and we should call __length_hint__() for every of them (unless found a one with not implemented __length_hint__()). This can slow down the execution for short sequences.

It is impossible to implement reasonable filter.__length_hint__(), because the length of resulting sequence can be from 0 to the length of the input sequence, and returning the maximal value would be not correct.