It is summing but being used for counting (in imitation of the same style from APL) via punning on True/False as 1/0. Not what actually happens but conceptuall

It is summing but being used for counting (in imitation of the same style from APL) via punning on True/False as 1/0. Not what actually happens but conceptually: ages = [17, 13, 18, 30, 12] sum(age > 17 for age in ages) => sum([False, False, True, True, False]) => sum([0, 0, 1, 1, 0]) => 2 # via conventional summing Since True and False are 1 and 0 for arithmetic in Python, this is just a regular sum which also happens to produce a count.