Convert a "dotted" dictionary to a nested one

dotted_to_nested.py

from functools import reduce
from typing import Any
from typing import Dict


def merge(a, b, path=None):
    """ Merge dictionary a into b (in-place)"""
    # source: https://stackoverflow.com/a/7205107/592289
    if path is None: path = []
    for key in b:
        if key in a:
            if isinstance(a[key], dict) and isinstance(b[key], dict):
                merge(a[key], b[key], path + [str(key)])
            elif a[key] == b[key]:
                pass # same leaf value
            else:
                raise Exception('Conflict at %s' % '.'.join(path + [str(key)]))
        else:
            a[key] = b[key]
    return a


def dotted_to_nested(dotted_dict: Dict[str, Any]) -> Dict[Any, Any]:
    """
    Convert a dictionary with "dotted" keys to a nested one. 

        >>> dotted_dict = {
            "AAA.aaa.111": 1,
            "AAA.aaa.222": 2,
            "AAA.bbb.111": 3,
            "AAA.bbb.333": 4,
            "BBB.aaa": 5,
            "CCC": 6,
            "DDD.ddd.999": 7,
        }
        >>> dotted_to_nested(dotted_dict)
        {'AAA': {'aaa': {'111': 1, '222': 2}},
        'BBB': {'aaa': 5},
        'CCC': 6,
        'DDD': {'ddd': {'999': 7}}}

    """
    # adapted from: https://stackoverflow.com/a/40401961/592289
    list_of_dicts = []
    for key, value in dotted_dict.items():
        row_dict = {}
        for i, subkey in enumerate(reversed(key.split("."))):
            if i == 0:
                row_dict = {subkey: value}
            else:
                row_dict = {subkey: row_dict}
        list_of_dicts.append(row_dict)
    nested_dict = reduce(merge, ({}, *list_of_dicts))
    return nested_dict


# example

dotted_dict = {
    "AAA.aaa.111": 1,
    "AAA.aaa.222": 2,
    "AAA.bbb.111": 3,
    "AAA.bbb.333": 4,
    "BBB.aaa": 5,
    "CCC": 6,
    "DDD.ddd.999": 7,
}
nested_dict = dotted_to_nested(dotted_dict)

from pprint import pprint
pprint(nested_dict)