A demonstration of the Elo rating system for 2-player competitions.

Elo.py

class Player:
	""" The "player" to be ranked """

	def __init__(self):
		""" Base rating of 1600 always """

		self.rating = 1600

	def getWinProb(self, opponent):
		""" Probablility that self will win against opponent """

		return 1.0 / ( 10**(( opponent.rating - self.rating)/400) + 1 )


def elo(winner, loser):
	
	# Constant for forumla
	K = 20

	# Winner's new rating
	a = winner.rating + (K * (1 - winner.getWinProb(loser)))

	# Loser's new rating
	b = loser.rating + (K * (0 - loser.getWinProb(winner)))

	# Set players' scores
	winner.rating = int(a + 0.5)
	loser.rating = int(b + 0.5)

alex = Player()
bob = Player()

while __name__ == "__main__":

	print "\n" + "=" * 10 + "\nAlex: ", alex.rating
	print "Bob : ", bob.rating, "\n" + "=" * 10

	i = raw_input("Should Alex or Bob win (a/b)? ")

	if "a" in i:
		elo(alex, bob)
	elif "b" in i:
		elo(bob, alex)