Transform a dictionary into a nested one and switch key <-> values in python

Question

I've got such a dict:

{'ab20': 'London', 'kn44': 'London', 'fe85': 'London', 'fg487': 'Paris', 'fe32': 'Paris'...}

It makes much more sence, if I change it into something like this:

{'London': ['ab20', 'kn44',  'fe85'], 'Paris': ['fg487', 'fe32'] ... }

What is the most efficient way to do this?

Answer 1

Use a defaultdict:

d  ={'ab20': 'London', 'kn44': 'London', 'fe85': 'London', 'fg487': 'Paris', 'fe32': 'Paris'}
from collections import defaultdict

result = defaultdict(list)
for key, val in d.items():
    result[val].append(key)
print(result)

Output:

{'London': ['ab20', 'kn44', 'fe85'], 'Paris': ['fg487', 'fe32']}