How Can I Efficiently Merge Nested Dictionaries in Python?

Merging Nested Dictionaries in Python

In Python, we often encounter scenarios where we need to combine multiple nested dictionaries. This task can be challenging, especially when the dictionaries have varying levels of depth and potential conflicts.

To merge dictionaries of dictionaries and maintain the hierarchy, we employ 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)])
            elif a[key] != b[key]:
                raise Exception('Conflict at ' + '.'.join(path + [str(key)]))
        else:
            a[key] = b[key]
    return a

This function accepts two dictionaries, a and b, along with an optional path parameter for tracking the current path in the merged dictionary. It iterates through each key in b and performs the following actions:

• If the key exists in a and both values are dictionaries, it recursively merges the two dictionaries and updates the path.
• If the key exists in a but the values are not dictionaries, it checks for conflicts. If a conflict is detected, an exception is raised.
• If the key does not exist in a, it adds it to a with the value from b.

Finally, the merged dictionary a is returned.

To merge multiple dictionaries, you can use reduce to combine all dictionaries into a single dictionary:

from functools import reduce
reduce(merge, [dict1, dict2, dict3...])

This operation will add the contents of all dictionaries to the first dictionary in the list.

Example:

dict1 = {1:{'a':{'A'}}, 2:{'b':{'B'}}}
dict2 = {2:{'c':{'C'}}, 3:{'d':{'D'}}}
print(merge(dict1, dict2))

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

Note that the resulting dictionary is stored in dict1.

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