Reverse / invert a dictionary by making keys values and values keys

Question

I am trying to reverse a dictionary by flipping the values with the keys

The output i got: {3: 'love', 2: 'self.py!'}

the output I want: {3: ['I', 'love'], 2: ['self.py']}

my code:

def inverse_dict(my_dict):
    inv_dict = {v: k for k, v in my_dict.items()}
    print(inv_dict)


def main():
    course_dict = {'I': 3, 'love': 3, 'self.py!': 2}
    inverse_dict(course_dict)


if __name__ == "__main__":
    main()

It's like took away the key and the value 'I': 3 someone knows why?

Does this answer your question? [Reverse / invert a dictionary mapping](https://stackoverflow.com/questions/483666/reverse-invert-a-dictionary-mapping) — iacob, Mar 16 '21 at 13:12
dictionary can have only unique keys and if you have multiple keys, it will be overwritten by the last value. — Krishna Chaurasia, Mar 16 '21 at 13:13
please read [What should I do when someone answers my question?](https://stackoverflow.com/help/someone-answers) — Vishal Singh, Mar 25 '21 at 05:15

score 2 · Answer 1 · answered Mar 16 '21 at 13:14

You can use the dictionary method setdefault for this. It works like this:
If the key is in the dictionary, return its value. If not, insert the key with a value of default(empty list in this case) and return default(list).

course_dict = {'I': 3, 'love': 3, 'self.py!': 2}
new_dict = {}
for key, value in course_dict.items():
    new_dict.setdefault(value, []).append(key)

print(new_dict)

Output:

{3: ['I', 'love'], 2: ['self.py!']}

score 2 · Answer 2 · answered Mar 16 '21 at 13:14

2

Not sure how you'd do this with a dict comprehension but using a default dict should be trivial

from collections import defaultdict

inv_dict = defaultdict(list)
for k,v in course_dict.items():
    inv_dict[v].append(k)

answered Mar 16 '21 at 13:14

Sayse

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ralf htp · Answer 3 · 2021-03-16T13:25:02.450

1

This code, copied from https://www.geeksforgeeks.org/python-program-to-swap-keys-and-values-in-dictionary/ (Method #2) checks for multiple values, like in your output

old_dict = {'A': 67, 'B': 23, 'C': 45, 'E': 12, 'F': 69, 'G': 67, 'H': 23} 
  
new_dict = {} 
for key, value in old_dict.items(): 
   if value in new_dict: 
       new_dict[value].append(key) 
   else: 
       new_dict[value]=[key]

edited Mar 16 '21 at 13:25

answered Mar 16 '21 at 13:19

ralf htp

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score 0 · Answer 4 · answered Mar 16 '21 at 13:20

I think the title of your question does not reflect exactly what you want to do according to the expected output you provided.

Indeed, instead of reversing / inverting a dictionary you want to group the keys per values.

This should do the job (I did not change the name of the function but you might want to since it can be misleading):

from collections import defaultdict

def inverse_dict(input_dict):
    inversed = defaultdict(list)
    for k, v in input_dict.items():
       inversed[v].append(k)
    
    return dict(inversed)

The output is the following:

inverse_dict({'I': 3, 'love': 3, 'self.py!': 2})
>>> {3: ['I', 'love'], 2: ['self.py!']}

Reverse / invert a dictionary by making keys values and values keys

4 Answers4