No! Never do this in Python. You are making the flattened list by continually concatenating the smaller lists. Each concatenation creates the new bigger list f

No! Never do this in Python. You are making the flattened list by continually concatenating the smaller lists. Each concatenation creates the new bigger list from scratch; the flattened list does not grow dynamically. This is quadratic-performance bad. Use `list(itertools.chain.from_iterable(...))` instead.