How to Perform a Deep Merge of Nested Dictionaries in Python?

Deep Merge of Dictionaries of Dictionaries in Python

Merging multiple dictionaries with nested keys can be a challenging task. Let's consider the following example:

dict1 = {
    1: {"a": {"A"}},
    2: {"b": {"B"}}
}

dict2 = {
    2: {"c": {"C"}},
    3: {"d": {"D"}}
}

The desired output is:

dict3 = {
    1: {"a": {"A"}},
    2: {"b": {"B"}, "c": {"C"}},
    3: {"d": {"D"}}
}

To achieve this deep merge, we can utilize a recursive function:

def merge(a: dict, b: dict, path=[]):
    for key in b:
        if key in a:
            if isinstance(a[key], dict) and isinstance(b[key], dict):
                merge(a[key], b[key], path + [str(key)])
            else:
                a[key] = b[key]
        else:
            a[key] = b[key]

    return a

This function takes two dictionaries, a and b, and a path that keeps track of the current location in the nested structure. It iterates through the keys in b and checks if the corresponding key exists in a. If the values at that key are both dictionaries, it calls the merge function recursively to merge those sub-dictionaries. Otherwise, it updates the value in a with the value from b.

For the example dicts above, the merge function would produce the desired result:

print(merge(dict1, dict2))

The output would be:

{
    1: {'a': {'A'}},
    2: {'b': {'B'}, 'c': {'C'}},
    3: {'d': {'D'}}
}

Note: This function mutates the first dictionary, a. If you wish to preserve its contents, you can make a copy before merging, e.g., merge(dict(a), b).

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