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.

I like some of @CMeza99's changes to @DomWeldon's implementation, but neither version respects/merges lists, so I hybridized the two and added a few things. Now lists will be merged, checks and raises an exception if both dicts have a key in common but different data types, and the method takes 2 or more dicts to merge instead of just two.

import collections.abc


def dict_merge(*args, add_keys=True):
    assert len(args) >= 2, "dict_merge requires at least two dicts to merge"
    rtn_dct = args[0].copy()
    merge_dicts = args[1:]
    for merge_dct in merge_dicts:
        if add_keys is False:
            merge_dct = {key: merge_dct[key] for key in set(rtn_dct).intersection(set(merge_dct))}
        for k, v in merge_dct.items():
            if not rtn_dct.get(k):
                rtn_dct[k] = v
            elif k in rtn_dct and type(v) != type(rtn_dct[k]):
                raise TypeError(f"Overlapping keys exist with different types: original is {type(rtn_dct[k])}, new value is {type(v)}")
            elif isinstance(rtn_dct[k], dict) and isinstance(merge_dct[k], collections.abc.Mapping):
                rtn_dct[k] = dict_merge(rtn_dct[k], merge_dct[k], add_keys=add_keys)
            elif isinstance(v, list):
                for list_value in v:
                    if list_value not in rtn_dct[k]:
                        rtn_dct[k].append(list_value)
            else:
                rtn_dct[k] = v
    return rtn_dct

Word of warning: the snippet is licensed under GPL

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.

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.