Simple Python Elo rating

Very simple Python implementation of the Elo rating system.

It contains two functions: expected(A, B), to calculate the expected score of A in a match against B; and elo(...) to calculate the new score for each player.

Usage example

(Example from Wikipedia)

In a five-round tournament, player A, with a rating of 1613, plays against opponents with the following ratings: 1609, 1477, 1388, 1586, 1720.

The expected score is therefore:

from elo import expected

exp  = expected(1613, 1609)
exp += expected(1613, 1477)
exp += expected(1613, 1388)
exp += expected(1613, 1586)
exp += expected(1613, 1720)

# exp == 2.867

A lost match #1, draw match #2, wins #3 and #4 and loses #5. Therefore the player's actual score is (0 + 0.5 + 1 + 1 + 0) = 2.5.

We can now use this to calculate the new Elo rating for A:

from elo import elo

elo(1613, 2.867, 2.5, k=32)  # 1601