Euclidean distance

critics = {
    'Lisa':{'Lady':2.5,'Snak':3.5,'Just':3.0,'Superman':3.5,'Dupree':2.5,'Night':3.0},
    'Gene':{'Lady':3.0,'Snak':3.5,'Just':1.5,'Superman':5.0,'Dupree':3.5,'Night':3.0},
    'Michael':{'Lady':2.5,'Snak':3.0,'Superman':3.5,'Night':4.0},
    'Claudia':{'Snak':3.5,'Just':3.0,'Superman':4.0,'Dupree':2.5,'Night':4.5},
    'Mick':{'Lady':3.0,'Snak':4.0,'Just':2.0,'Superman':3.0,'Dupree':2.0,'Night':3.0},
    'Jack':{'Lady':3.0,'Snak':4.0,'Just':3.0,'Superman':5.0,'Dupree':3.5,'Night':3.0},
    #'Toby':{'Snak':4.5,'Superman':4.0,'Dupree':1.0}
    'Toby':{}
}
from math import sqrt
def sim_distance(prefs,person1,person2):
    si={}
    for item in prefs[person1]:
        if item in prefs[person2]:
            si[item]=1
    if len(si)==0:
        return 0
    sum_of_squares=sum([pow(prefs[person1][item]-prefs[person2][item],2) for item in prefs[person1] if item in prefs[person2]])
    return 1/(1+sqrt(sum_of_squares))

print (sim_distance(critics,'Lisa','Gene'))
print (sim_distance(critics,'Lisa','Toby'))
print (sim_distance(critics,'Lisa','Michael'))