How Can I Recursively Merge Multi-Level Dictionaries in Python?

Merging Multi-Level Dictionaries Deeply

In Python, merging dictionaries recursively while maintaining an unknown depth of nesting can be a challenge. Here's a solution that addresses this issue:

def deep_merge(a: dict, b: dict, path=[]):
    for key in b:
        if key in a:
            if isinstance(a[key], dict) and isinstance(b[key], dict):
                deep_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 takes two dictionaries as input and recursively merges their contents. It maintains a path to track the current nesting level to provide more precise error messages in case of conflicts.

Example:

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

print(result)
# {1:{'a':{'A'}}, 2:{'b':{'B'}, 'c':{'C'}}, 3:{'d':{'D'}}}

Note that deep_merge mutates the first argument, which is why result contains the merged dictionary.

To merge multiple dictionaries without modifying any of them, pass them as keyword arguments:

from functools import reduce
result = reduce(deep_merge, [dict1, dict2, ...], {})

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