Python Group and aggregate unidirectionally a list of dictionaries by multiple keys

Am building a tree selector, I need to structure my data like a tree of grouped items. I have bellow input which is a list of dictionaries.

data = [
        {'region': 'R1', 'group': 'G1', 'category': 'C1', 'item': 'I2'},
        {'region': 'R1', 'group': 'G1', 'category': 'C1', 'item': 'I1'},
        {'region': 'R1', 'group': 'G2', 'category': 'C2', 'item': 'I3'},
        {'region': 'R2', 'group': 'G1', 'category': 'C1', 'item': 'I1'},
        {'region': 'R2', 'group': 'G2', 'category': 'C2', 'item': 'I3'},
        {'region': 'R2', 'group': 'G2', 'category': 'C2', 'item': 'I4'},
        {'region': 'R2', 'group': 'G2', 'category': 'C3', 'item': 'I5'},
    ]

I want to get the following output

result = {
  "regions": [
    {
      "name": "R1",
      "groups": [
        {
          "name": "G1",
          "categories": [
            {"name": "C1","items": [{ "name": "I2"},{"name": "I1"}]}
          ]
        },
        {
          "name": "G2",
          "categories": [
            {"name": "C2", "items": [{"name": "I3"}]}
          ]
        }
      ]
    },
    {
      "name": "R2",
      "groups": [
        {
          "name": "G1",
          "categories": [
            {"name": "C1","items": [{"name": "I1"}]}
          ]
        },
        {
          "name": "G2",
          "categories": [
            {"name": "C2","items": [{"name": "I3"},{"name": "I4"}]},
            {"name": "C3", "items": [{"name": "I5"}]}
          ]
        }
      ]
    }
  ]
}

After some researches I come up with this solution

from collections import OrderedDict

d = OrderedDict()
    for aggr in data:
        d.setdefault(
            key=(aggr['region'], aggr['group'], aggr['category']),
            default=list()
        ).append({"name": aggr['item']})
    d1 = OrderedDict()
    for k, v in d.items():
        d1.setdefault(
            key=(k[0], k[1]),
            default=list()
        ).append({"name": k[2], "items": v})
    d2 = OrderedDict()
    for k, v in d1.items():
        d2.setdefault(
            key=k[0],
            default=list()
        ).append({"name": k[1], "categories": v})
    result = {"regions": [{"name": k, "groups": v} for k, v in d2.items()]}

It's working but I believe it's not the most pythonic solution. I did not manage to simplify it.

Any help to propose another solution or improvement on above codes will be appreciated

As long as the items are sorted, like in your example, you could use groupby from itertools in a recursive function, like:

from itertools import groupby
from operator import itemgetter

def plural(word):
    return f"{word}s" if word[-1] != 'y' else f"{word[:-1]}ies"

def grouping(records, *keys):
    if len(keys) == 1:
        return [{"name": record[keys[0]]} for record in records]
    return [
        {"name": key, plural(keys[1]): grouping(group, *keys[1:])}
        for key, group in groupby(records, itemgetter(keys[0]))
    ]

result = {"regions": grouping(data, "region", "group", "category", "item")}

If the sorting isn't guaranteed, then you could adjust grouping in the following way

def grouping(records, *keys):
    if len(keys) == 1:
        return [{"name": record[keys[0]]} for record in records]
    key_func = itemgetter(keys[0])
    records = sorted(records, key=key_func)
    return [
        {"name": key, plural(keys[1]): grouping(group, *keys[1:])}
        for key, group in groupby(records, key_func)
    ]

or sort the data beforehand

keys = ["region", "group", "category", "item"]
data = sorted(data, key=itemgetter(*keys))
result = {"regions": grouping(data, *keys)}

Result of first version for data as provided in the question:

result = {
   "regions": [
      {
         "name": "R1",
         "groups": [
            {
               "name": "G1",
               "categories": [
                  {"name": "C1", "items": [{"name": "I2"}, {"name": "I1"}]
                  }
               ]
            },
            {
               "name": "G2",
               "categories": [
                  {"name": "C2", "items": [{"name": "I3"}]}
               ]
            }
         ]
      },
      {
         "name": "R2",
         "groups": [
            {
               "name": "G1",
               "categories": [
                  {"name": "C1", "items": [{"name": "I1"}]}
               ]
            },
            {
               "name": "G2",
               "categories": [
                   {"name": "C2", "items": [{"name": "I3"}, {"name": "I4"}]},
                   {"name": "C3", "items": [{"name": "I5"}]}
               ]
            }
         ]
      }
   ]
}