A lazy pipeline over lines in Python
Python interview exercise on Iterators & Generators, difficulty 3 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 error_messages(lines), a generator over log lines shaped like "LEVEL message". Yield the message of each line whose level is ERROR, stripped. It must read only as far as it is asked to: the tests feed it a source that blows up if read too far.
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.