How Can I Efficiently Merge Python Dictionaries with Duplicate Keys?

Merging Dictionaries with Duplicate Keys

In Python, dictionaries are used to store collections of key-value pairs. When dealing with multiple dictionaries with duplicate keys, merging them while maintaining the associated values can be a common challenge.

One efficient method for achieving this is through the use of the collections.defaultdict from the Python standard library. This specialized dictionary allows for value initialization with a default factory, such as a list, for non-existent keys.

Consider the following sample dictionaries:

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

To merge these dictionaries, we can initialize a defaultdict with a default value of an empty list:

dd = defaultdict(list)

Next, we iterate over each dictionary in the sequence along with their key-value pairs:

for d in (d1, d2):
    for key, value in d.items():
        dd[key].append(value)

In this loop, for each key encountered, we append the corresponding value to the default list. This approach ensures that all duplicate keys are handled with their associated values.

As a result, the dd dictionary will contain merged values corresponding to duplicate keys:

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

This method is particularly useful when dealing with large sets of dictionaries or when there can be an arbitrary number of input dictionaries. It efficiently merges all duplicate keys while preserving their values in a consolidated output dictionary.

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