#!/usr/bin/env python
# encoding: utf-8

from collections import defaultdict


word2value = {'dog': 15, 'dogs': 20}
word2stem  = {'dog': 'dog', 'dogs': 'dog'}

stem2value = defaultdict(int)


for word, stem in word2stem.iteritems():
    stem2value[stem] += word2value[word]

for word in word2value:
    word2value[word] = stem2value[ word2stem[word] ]

print word2value

# {'dog': 35, 'dogs': 35}