Reverse / invert a dictionary mapping

Question

Given a dictionary like so:

my_map = {\'a\': 1, \'b\': 2}

How can one invert this map to get:

inv_map = {1: \'a\', 2: \'b\'         


        

Answer 1

We can also reverse a dictionary with duplicate keys using defaultdict:

from collections import Counter, defaultdict

def invert_dict(d):
    d_inv = defaultdict(list)
    for k, v in d.items():
        d_inv[v].append(k)
    return d_inv

text = 'aaa bbb ccc ffffd aaa bbb ccc aaa' 
c = Counter(text.split()) # Counter({'aaa': 3, 'bbb': 2, 'ccc': 2, 'ffffd': 1})
dict(invert_dict(c)) # {1: ['ffffd'], 2: ['bbb', 'ccc'], 3: ['aaa']}  

See here:

This technique is simpler and faster than an equivalent technique using dict.setdefault().