doc_a: [word_w, word_x, word_y]
  doc_b: [word_x, word_z]
  doc_c: [word_y]

  word_w -> [doc_a]
  word_x -> [doc_a, doc_b]
  word_y -> [doc_a, doc_c]
  word_z -> [doc_b]

doc_a = {'id': 'a', 'words': ['word_w', 'word_x', 'word_y']} 
doc_b = {'id': 'b', 'words': ['word_x', 'word_z']} 
doc_c = {'id': 'c', 'words': ['word_y']} 
 
docs = [doc_a, doc_b, doc_c] 
indices = dict() 
 
for doc in docs: 
  for word in doc['words']: 
    if word not in indices: 
      indices[word] = [] 
    indices[word].append(doc['id']) 
 
print indices

if word not in indices: 
  indices[word] = []

for doc in docs: 
  for word in doc['words']: 
    indices. setdefault(word, []) .append(doc['id'])

def setdefault(self, key, default_factory): 
  if key not in self: 
    self[key] = default_factory() 
  return self[key]

for doc in docs: 
  for word in doc['words']: 
    indices.setdefault(word, list ).append(doc['id'])

def word_count(words): 
  count = dict() 
  for word in words: 
    count.setdefault(word, 0) += 1
  return count 
 
print word_count(['hiiragi', 'kagami', 'hiiragi', 'tukasa', 'yosimizu', 'kagami'])

from collections import defaultdict 
 
doc_a = {'id': 'a', 'words': ['word_w', 'word_x', 'word_y']} 
doc_b = {'id': 'b', 'words': ['word_x', 'word_z']} 
doc_c = {'id': 'c', 'words': ['word_y']} 
 
docs = [doc_a, doc_b, doc_c] 
indices = defaultdict(list) 
 
for doc in docs: 
  for word in doc['words']: 
    indices[word].append(doc['id']) 
 
print indices 
 
def word_count(words): 
  count = defaultdict(int) 
  for word in words: 
    count[word] += 1
  return count 
 
print word_count(['hiiragi', 'kagami', 'hiiragi', 'tukasa', 'yosimizu', 'kagami'])