https://medium.com/@mmas/hadoop-streaming-practical-introduction-to-mapreduce-with-python-772bf08070c5

word_counter_reducer.py

#!/usr/bin/env python

import sys


last_k = None
last_v = 0

for line in sys.stdin:
    k, v = line.strip().split('\t')
    v = int(v)

    if last_k == k:
        last_v += v
    else:
        if last_k:
            print '%s\t%s' % (last_k, last_v)
        last_k = k
        last_v = v

if last_k == k:
    print '%s\t%s' % (last_k, last_v)