deep merge nested dictionaries in Python

dicts_merge.py

"""
Deep merging two dictionaries with any level of nesting.
Based on solution found at https://stackoverflow.com/a/71024248/7521470
Fixed a bug + mutability issue
"""

import copy
from unittest import TestCase
import pprint

pp = pprint.PrettyPrinter(indent=2)


def merge(source: dict, destination: dict) -> dict:
    """ copies the items FROM SOURCE TO DESTINATION """
    destination_copy = copy.deepcopy(destination)
    for key, value in source.items():
        destination_key_value = destination_copy.get(key)
        if isinstance(value, dict) and isinstance(destination_key_value, dict):
            destination_copy[key] = merge(value, destination_copy.setdefault(key, {}))
        else:
            destination_copy[key] = value
    return destination_copy


class ModuleFuncsTests(TestCase):

    def test_merge_dicts(self):
        print('--test_merge_dicts--')
        sub_cases = [
            ({'A': 'B'}, {'A': 'D'}, {'A': 'B'}),
            ({'A': True}, {'A': False}, {'A': True}),
            ({'A': {'B': 'C'}}, {'A': False}, {'A': {'B': 'C'}}),
            ({'first': {'all_rows': {'pass': 'dog', 'number': '1'}}},
             {'first': {'all_rows': {'fail': 'cat', 'number': '5'}}},
             {'first': {'all_rows': {'pass': 'dog', 'fail': 'cat', 'number': '1'}}}),
            ({'b': {'c': 1}}, {'b': 1}, {'b': {'c': 1}}),
            ({}, {'A (1)': {'B (2)': {'C (3)': [{'smth': 48, 'qwerty': 2}]}}},
             {'A (1)': {'B (2)': {'C (3)': [{'smth': 48, 'qwerty': 2}]}}}),
            ({'A (1)': {'B (2)': {'D (3)': [{'bla-bla-bla': 1}]}}},
             {'A (1)': {'D (3)': [{'bla-bla-bla': 1}]}},
             {'A (1)': {'B (2)': {'D (3)': [{'bla-bla-bla': 1}]}, 'D (3)': [{'bla-bla-bla': 1}]}})
        ]
        for s in sub_cases:
            print('\n')
            pp.pprint(s)
            source, destination, control = s
            res = merge(source, destination)
            print(res)
            with self.subTest(i=s):
                self.assertDictEqual(res, control)