prize13-6, Exercise 3: A rating upset

Model answer (check the details):

    print(round(expected(1800, 2000), 2))        # 0.24
    print(update(1800, 2000, 1))                 # about (1815.2, 1984.8)

The 1800 player is expected to score about 0.24 per game against the 2000 player. If he wins (score 1), with K = 20 he gains 20 x (1 - 0.24) = about 15.2 points, so his new rating is about 1815.2, and the loser drops by the same amount to about 1984.8.

An upset is rewarded more than an expected win would be: a win between equal players moves each rating by only 10 points. The ratings change by the same total, so the system is zero-sum between the two.

Care: K varies by federation and rating; a different K gives different numbers.