Inverting a dictionary when some of the original values are identical

Question

Say I have a dictionary called word_counter_dictionary that counts how many words are in the document in the form {\'word\' : number}. For example,

Answer 1

A defaultdict is perfect for this

word_counter_dictionary = {'first':1, 'second':2, 'third':3, 'fourth':2}
from collections import defaultdict

d = defaultdict(list)
for key, value in word_counter_dictionary.iteritems():
    d[value].append(key)

print(d)

Output:

defaultdict(, {1: ['first'], 2: ['second', 'fourth'], 3: ['third']})