How Can I Efficiently Merge Dictionaries with Duplicate Keys and Collect Their Values?

Merging Dictionaries for Duplicate Keys

When working with multiple dictionaries containing overlapping keys, it becomes necessary to efficiently merge them while collecting values associated with those keys. This article explores a solution to combine and collect values from matching keys within a collection of dictionaries.

Problem Statement

Given several dictionaries like:

d1 = {key1: x1, key2: y1}
d2 = {key1: x2, key2: y2}

The goal is to obtain a merged result as a new dictionary, where each key holds a tuple of values from the original dictionaries:

d = {key1: (x1, x2), key2: (y1, y2)}

Solution

The solution utilizes the collections.defaultdict to create a dictionary with default values as mutable lists. This allows the accumulation of values for each key:

from collections import defaultdict

d1 = {1: 2, 3: 4}
d2 = {1: 6, 3: 7}

dd = defaultdict(list)

for d in (d1, d2): # Input dictionaries can be iterated over here
    for key, value in d.items():
        dd[key].append(value)

The final merged dictionary dd contains each key mapped to a list of values from the input dictionaries.

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