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

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)}