bpo-34659: Adds initial kwarg to itertools.accumulate() by lisroach · Pull Request #9345 · python/cpython (original) (raw)

This will make a subtle difference between accumulate() and reduce(), and also max(), iter(), next(), getattr(), etc which accept an optional argument, but distinguish None from no argument. I think this will be a bug magnet.

As for the pure python version in the docs, the pure python version of reduce() uses None as a sentinel.