Recursive dictionary merge in Python

dict_merge.py

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]

zLICENSE

# Copyright 2024 Paul Durivage 
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

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.