Two utils functions to generate iterative settable dicts.

iterative_dict.py

from typing import List
from collections import defaultdict

def nested_set(nested_dict: defaultdict, path: List[str]) -> defaultdict:
    '''
    A function that sets a value to a defaultdict given a path of keys.
    
    Example:
        d = defaultdict(dict)
        d = nested_set(d, ['hello', 'ciao', 3])
        print(d)
        >>> { 'hello' : { 'ciao' : 3 } }
    '''
    [key, *rest] = path
    if len(rest) == 1:
        nested_dict[key] = rest[0]
        return nested_dict

    return nested_set(nested_dict[key], rest)


def json_from_arr(arr):
    nest = lambda: defaultdict(nest)
    json = nest()

    for row in arr:
        nested_set(json, row)

    return json


if __name__ == '__main__':
    data = [
        ["BLUE", "XXL", 'H3', 98],
        ["BLUE", "XL", 'H4', 97],
        ["BLUE", "L", 99],
        ["BLUE", "M", 103],
        ["PINK", "XXL", 104],
        ["PINK", "XL", 103],
        ["PINK", "L", 102],
        ["PINK", "M", 100],
        ["RED", "XXL", 99],
        ["RED", "XL", 102],
        ["RED", "L", 109],
        ["RED", "M", 95],
    ]

    nested_dict = json_from_arr(data)