How to reverse a dictionary that has repeated values

Question

I have a dictionary with almost 100,000 (key, value) pairs and the majority of the keys map to the same values. For example:

mydict =  {\'a\': 1, \'c\': 2, \'         


        

Answer 1

reversed_dict = collections.defaultdict(list)
for key, value in dict_.iteritems():
  reversed_dict[value].append(key)