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)

Answer 2

Using collections.defaultdict:

from collections import defaultdict

reversed_dict = defaultdict(list)
for key, value in mydict.items():
    reversed_dict[value].append(key)

Answer 3

reversed_dict = {}
for key, value in mydict.items():
    reversed_dict.setdefault(value, [])
    reversed_dict[value].append(key)

Answer 4

for k,v in dict.iteritems():
    try:
      reversed_dict[v].append(k)
    except KeyError:
       reversed_dict[v]=[k]

Answer 5

I think you're wasting a few cycles by replacing a key with the same key again and again...

reversed_dict = {}
for value in mydict.values():
    if value not in reversed_dict.keys(): #checking to be sure it hasn't been done.
        reversed_dict[value] = []
        for key in mydict.keys():
            if mydict[key] == value:
                if key not in reversed_dict[value]: reversed_dict[value].append(key)

Answer 6

Using itertools.groupby:

from operator import itemgetter
from itertools import groupby

snd = itemgetter(1)

def sort_and_group(itr, f):
    return groupby(sorted(itr, key=f), f)

mydict =  {'a': 1, 'c': 2, 'b': 1, 'e': 2, 'd': 3, 'h': 1, 'j': 3}
reversed_dict = {number: [char for char,_ in v] 
                 for number, v in sort_and_group(mydict.items(), snd)}