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

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)