How to reverse a dictionary that has repeated values

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

Using collections.defaultdict:

from collections import defaultdict

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

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