Recursive dictionary merge in Python

dict_merge.py

# Copyright 2016-2022 Paul Durivage
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import collections

def dict_merge(dct, merge_dct):
    """ Recursive dict merge. Inspired by :meth:``dict.update()``, instead of
    updating only top-level keys, dict_merge recurses down into dicts nested
    to an arbitrary depth, updating keys. The ``merge_dct`` is merged into
    ``dct``.

    :param dct: dict onto which the merge is executed
    :param merge_dct: dct merged into dct
    :return: None
    """
    for k, v in merge_dct.iteritems():
        if (k in dct and isinstance(dct[k], dict) and isinstance(merge_dct[k], dict)):  #noqa
            dict_merge(dct[k], merge_dct[k])
        else:
            dct[k] = merge_dct[k]

If you find this online and are looking to merge two dictionaries, please see the new dictionary merge features of Python 3.9 as discussed extensively in PEP 584.

I fail to see how this help

In some of the solutions here the returned dict contains references to the input dicts. This could cause some serious bugs.

Testcase:

a = {}
b = {"a": {"b": 1, "c": 2}}
a = deep_merge(a, b)
assert a == b
b["a"]["b"] = 5
assert a != b

My version which passes this test (MIT license):

from copy import deepcopy

def deep_merge(a: dict, b: dict) -> dict:
    result = deepcopy(a)
    for bk, bv in b.items():
        av = result.get(bk)
        if isinstance(av, dict) and isinstance(bv, dict):
            result[bk] = deep_merge(av, bv)
        else:
            result[bk] = deepcopy(bv)
    return result

@tfeldmann I am pretty sure that this:

        av = result.get("k")

should be this:

        av = result.get(bk)

Yes of course. Corrected.

Everything works fine if merge_dict is empty. Your version doesn't return a copy but the base dict itself, so that could lead to bugs.