Python - Keys associated with Values in Dictionary

Given a dictionary in Python, our task is to reverse the mapping, i.e., create a new dictionary where each value points to the keys it belongs to. This operation is useful in data organization, analytics, and web applications.

Example:

Input: {'abc': [10, 30], 'bcd': [30, 40, 10]}
Output: {10: ['abc', 'bcd'], 30: ['abc', 'bcd'], 40: ['bcd']}

Using defaultdict() + Loop

defaultdict from the collections module simplifies grouping by automatically initializing missing keys.

from collections import defaultdict

d = {'gfg': [1, 2, 3], 'is': [1, 4], 'best': [4, 2]}
res = defaultdict(list)
for key, vals in d.items():
    for v in vals:
        res[v].append(key)
print(dict(res))

{1: ['gfg', 'is'], 2: ['gfg', 'best'], 3: ['gfg'], 4: ['is', 'best']}

Explanation:

• defaultdict(list): creates a dictionary where each key automatically starts with an empty list.
• res[v].append(key): appends the current key to the list of keys associated with value v.

Using Dictionary and Loops

This method builds the result dictionary manually by checking key existence.

d = {'gfg': [1, 2, 3], 'is': [1, 4], 'best': [4, 2]}

res = {}
for key, vals in d.items():
    for v in vals:
        if v in res:
            res[v].append(key)
        else:
            res[v] = [key]
print( res)

{1: ['gfg', 'is'], 2: ['gfg', 'best'], 3: ['gfg'], 4: ['is', 'best']}

Explanation:

• for key, vals in d.items(): iterates over each key–value pair.
• if v in res: checks if the value already exists in the result dictionary.
• res[v].append(key): adds the current key to the existing list of keys for that value.

Using setdefault()

setdefault() method helps avoid key existence checks.

d = {'gfg': [1, 2, 3], 'is': [1, 4], 'best': [4, 2]}

res = {}
for key, vals in d.items():
    for v in vals:
        res.setdefault(v, []).append(key)

print( res)

{1: ['gfg', 'is'], 2: ['gfg', 'best'], 3: ['gfg'], 4: ['is', 'best']}

Explanation:

• for key, vals in d.items(): Iterates through each key-value pair in the dictionary.
• for v in vals: For every value, adds its corresponding key to res.
• res.setdefault(v, []).append(key): Uses setdefault() to handle missing keys efficiently.

Related Articles:

• Python | Ways to invert mapping of dictionary
• Inverse Dictionary Values List - Python