KeyAvgreducer.py

#!/usr/bin/python
#Reducer.py
import sys

MovieID_Rating = {}

#Partitoner
for line in sys.stdin:
    line = line.strip()
    MovieID, Rating = line.split('\t')

    if MovieID in MovieID_Rating:
        MovieID_Rating[MovieID].append(int(Rating))
    else:
        MovieID_Rating[MovieID] = []
        MovieID_Rating[MovieID].append(int(Rating))

#Reducer
for MovieID in MovieID_Rating.keys():
    ave_Rating = sum(MovieID_Rating[MovieID])*1.0 / len(MovieID_Rating[MovieID])
    print '%s\t%s'% (MovieID, ave_Rating)