Comment inverser un dictionnaire qui a des valeurs répétées

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

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

Je pense que vous gaspillez quelques cycles en remplaçant une clé avec la même clé encore et encore ...

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)

Utilisation de iTERTOOLS.GROUPBY CODE>:

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)}

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