How Can I Efficiently Merge Multiple Dictionaries with Potentially Missing Keys?

Efficiently Aggregating Data from Multiple Dictionaries

When working with multiple dictionaries, the need may arise to merge their data, collecting values for matching keys into a new dictionary. This task poses a challenge when the dictionaries may contain keys that are missing in others.

To address this challenge effectively, we can utilize the defaultdict from the collections module. Here's how it works:

from collections import defaultdict

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

dd = defaultdict(list)

for d in (d1, d2): # Include all dictionaries here
    for key, value in d.items():
        dd[key].append(value)

This code iterates over each dictionary, adding each key-value pair to the defaultdict. The defaultdict automatically initializes missing keys with an empty list. Thus, when a key is encountered in a dictionary but not in the previous ones, a new list is created for its values.

The final result obtained in dd is a defaultdict where each key corresponds to a list of values collected from all the input dictionaries.

print(dd) # Result: defaultdict(<type 'list'>, {1: [2, 6], 3: [4, 7]})

This solution ensures efficient and comprehensive data aggregation from multiple dictionaries, handling even cases with missing keys.

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