Python – Inverse Dictionary Values List

Inverting a dictionary means swapping keys and values. When the original dictionary has lists as values, we create a new dictionary where each item in those lists becomes a key, and the original keys become the values.

Using defaultdict

The defaultdict from collections module automatically creates empty lists for new keys ?

from collections import defaultdict

my_dict = {13: [12, 23], 22: [31], 34: [21], 44: [52, 31]}
print("The original dictionary is:")
print(my_dict)

inverted_dict = defaultdict(list)

for key, values in my_dict.items():
    for value in values:
        inverted_dict[value].append(key)

print("The inverted dictionary is:")
print(dict(inverted_dict))

The original dictionary is:
{13: [12, 23], 22: [31], 34: [21], 44: [52, 31]}
The inverted dictionary is:
{12: [13], 23: [13], 31: [22, 44], 21: [34], 52: [44]}

Using Regular Dictionary

Without defaultdict, we need to check if a key exists before appending ?

my_dict = {13: [12, 23], 22: [31], 34: [21], 44: [52, 31]}
print("The original dictionary is:")
print(my_dict)

inverted_dict = {}

for key, values in my_dict.items():
    for value in values:
        if value in inverted_dict:
            inverted_dict[value].append(key)
        else:
            inverted_dict[value] = [key]

print("The inverted dictionary is:")
print(inverted_dict)

The original dictionary is:
{13: [12, 23], 22: [31], 34: [21], 44: [52, 31]}
The inverted dictionary is:
{12: [13], 23: [13], 31: [22, 44], 21: [34], 52: [44]}

Using Dictionary Comprehension

A more compact approach using dictionary comprehension and setdefault() ?

my_dict = {13: [12, 23], 22: [31], 34: [21], 44: [52, 31]}
print("The original dictionary is:")
print(my_dict)

inverted_dict = {}
for key, values in my_dict.items():
    for value in values:
        inverted_dict.setdefault(value, []).append(key)

print("The inverted dictionary is:")
print(inverted_dict)

The original dictionary is:
{13: [12, 23], 22: [31], 34: [21], 44: [52, 31]}
The inverted dictionary is:
{12: [13], 23: [13], 31: [22, 44], 21: [34], 52: [44]}

How It Works

• We iterate through each key-value pair in the original dictionary

• For each list of values, we iterate through individual items

• Each item becomes a key in the new dictionary

• The original key is appended to the list of values for that new key

• Multiple original keys can map to the same value, creating lists in the result

Comparison

Conclusion

Dictionary inversion with list values creates a mapping where original values become keys and original keys become grouped values. Use defaultdict for cleaner code or setdefault() for a more compact solution.