json --- JSON エンコーダおよびデコーダ がだいたいわかる最小の使用例を示す https://docs.python.org/ja/3/library/json.html

example.jsonを次のように用意する

{
    "person1": {
        "id": 10,
        "name": "Gon",
        "type": "Kyoka",
        "weapon": "Ja Ja n Ken, Gu, Choki, Par"
    },
    "person2": {
        "id": 20,
        "name": "Killua",
        "type": "Henka",
        "weapon": "Jippu-jinrai, Kanmuru, Narukami"
    },
    "person3": {
        "id": 30,
        "name": "Kurapika",
        "type": "Gugenka",
        "weapon": "Judgement, Jail, etc chain."
    }
}

次の例ではこのexample.jsonを読み込んでforを使ってkey, valueを表示し、新しいエントリを追加し、それをexample-out.jsonに書き出している。

#!/usr/bin/env python3

import os
import json

filename = 'example.json'

with open('example.json', 'r') as file:
    obj = json.load(file)
    for key in obj:
        print(key, obj[key])

obj["person4"] = {
    "id": 40,
    "name": "Irumi",
    "type": "So-sa",
    "weapon": "Hari-ningyo"
}

name, ext = os.path.splitext(filename)
filename_out = os.path.join(name+'-out'+ext)

with open(filename_out, "w") as file:
    json.dump(obj, file, indent=4)

ありがたいことに、JSONの記法とPythonのdictは表記が同じで書ける。indent=4でヒューマンリーダブルなJSONを書き出せる。結果は次のようになる。

{
    "person1": {
        "id": 10,
        "name": "Gon",
        "type": "Kyoka",
        "weapon": "Ja Ja n Ken, Gu, Choki, Par"
    },
    "person2": {
        "id": 20,
        "name": "Killua",
        "type": "Henka",
        "weapon": "Jippu-jinrai, Kanmuru, Narukami"
    },
    "person3": {
        "id": 30,
        "name": "Kurapika",
        "type": "Gugenka",
        "weapon": "Judgement, Jail, etc chain."
    },
    "person4": {
        "id": 40,
        "name": "Irumi",
        "type": "So-sa",
        "weapon": "Hari-ningyo"
    }
}

以上！