#!/usr/local/bin/python
 # -*- coding: utf-8 -*-

from math import sqrt

# 1 / 相似部分的欧几里得距离平方和
def sim_distance(prefs, person1, person2) :
	shared_items = 0
	for item in prefs[person1]:
		if item in prefs[person2]: shared_items += 1;
	if shared_items == 0:
		return 0
	sum_square_distance = 0
	sum_square_distance = sum([ pow((prefs[person1][item]-prefs[person2][item]), 2)
								for item in prefs[person1] if item in prefs[person2] ])
	return 1/sqrt(sum_square_distance+1)

pref = {
	"jack" : {
		"chess" : 2,
		"football" : 3
	},
	"jelly" : {
		"chess" : 2
	}
}

print sim_distance(pref, "jack", "jelly")