Filter a stream, remembering the past in Python
Python interview exercise on Iterators & Generators, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write unique_everseen(iterable), a generator that yields each distinct item the first time it appears, in order. It must be lazy: it works on an endless input and yields an item as soon as it is seen.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.