Pythonで設定ファイル(JSON/YAML/INI)を読み書きする方法まとめ

# PyYAMLのインストール
python -m pip install PyYAML

# バージョン確認(必要なら)
python -c "import yaml; print(yaml.__version__)"

// config/config.json
{
  "app_name": "SampleApp",
  "debug": true,
  "retry": { "count": 3, "interval": 1.5 },
  "hosts": ["example.com", "api.example.com"],
  "message": "こんにちは"
}

# config/config.yaml
app_name: SampleApp
debug: true
retry:
  count: 3
  interval: 1.5
hosts:
  - example.com
  - api.example.com
message: "こんにちは"
enabled: on        # ← YAMLでは on は True と解釈されます(注意)
release_date: 2025-09-01  # ← 日付形式も date 型に推測される場合があります

; config/config.ini
[DEFAULT]
app_name = SampleApp
debug = true
retry_count = 3
retry_interval = 1.5

[server]
host = example.com
port = 8080
use_tls = yes
endpoint = https://%(host)s:%(port)s  ; 既定の補間で%(host)sなどを参照可能

# json_examples.py
from pathlib import Path
import json

BASE_DIR = Path(__file__).resolve().parent
CONFIG_PATH = BASE_DIR / "config" / "config.json"

def read_json_file():
    # ファイルから読み込む場合は json.load を使用
    with CONFIG_PATH.open(encoding="utf-8") as f:
        data = json.load(f)
    # データは通常 dict として得られます
    print("app_name:", data["app_name"])
    print("retry.count:", data["retry"]["count"])
    print("hosts:", data["hosts"])

def read_json_string():
    # 文字列からパースする場合は json.loads を使用
    json_text = CONFIG_PATH.read_text(encoding="utf-8")
    data = json.loads(json_text)
    print("message:", data["message"])

if __name__ == "__main__":
    read_json_file()
    read_json_string()

# json_examples.py (続き)
def write_json_file():
    src = json.loads(CONFIG_PATH.read_text(encoding="utf-8"))
    # 値を変更・追加
    src["debug"] = False
    src["last_updated"] = "2025-09-20"

    out_path = BASE_DIR / "config" / "out_config.json"
    # ensure_ascii=False で日本語など非ASCII文字をそのまま書き出し
    with out_path.open("w", encoding="utf-8") as f:
        json.dump(src, f, ensure_ascii=False, indent=2, sort_keys=True)

    # 文字列に整形して出力したいときは dumps
    pretty = json.dumps(src, ensure_ascii=False, indent=2)
    print(pretty)

if __name__ == "__main__":
    write_json_file()

from pathlib import Path
import json
from json import JSONDecodeError

BASE_DIR = Path(__file__).resolve().parent
CONFIG_PATH = BASE_DIR / "config" / "config.json"

DEFAULTS = {
    "app_name": "SampleApp",
    "debug": False,
    "retry": {"count": 3, "interval": 1.0},
    "hosts": ["localhost"],
}

def load_config_safe(path: Path):
    try:
        with path.open(encoding="utf-8") as f:
            return json.load(f)
    except FileNotFoundError:
        print("[WARN] 設定ファイルが見つかりません。既定値を使用します:", path)
        return DEFAULTS
    except JSONDecodeError as e:
        print(f"[ERROR] JSONの形式が不正です(行{e.lineno} 列{e.colno}): {e.msg}")
        print("[INFO] 既定値を使用します")
        return DEFAULTS.copy()

if __name__ == "__main__":
    cfg = load_config_safe(CONFIG_PATH)
    print("debug:", cfg["debug"])

python -m pip install PyYAML

# yaml_examples.py
from pathlib import Path
import yaml
import datetime as dt

BASE_DIR = Path(__file__).resolve().parent
YAML_PATH = BASE_DIR / "config" / "config.yaml"

def read_yaml():
    with YAML_PATH.open(encoding="utf-8") as f:
        data = yaml.safe_load(f)  # dictとして読み込まれる
    print("app_name:", data["app_name"])
    print("hosts:", data["hosts"])

    # YAMLは値を型推測します
    print("enabled:", data["enabled"], type(data["enabled"]))  # True と解釈される
    print("release_date:", data["release_date"], type(data["release_date"]))  # date型の可能性

if __name__ == "__main__":
    read_yaml()

# yaml_examples.py (続き)
def write_yaml():
    with YAML_PATH.open(encoding="utf-8") as f:
        data = yaml.safe_load(f)

    data["debug"] = False
    data["notes"] = "日本語のコメントもOK"

    out_path = BASE_DIR / "config" / "out_config.yaml"
    # allow_unicode=True で日本語をそのまま書き出し
    with out_path.open("w", encoding="utf-8") as f:
        yaml.safe_dump(
            data,
            f,
            allow_unicode=True,      # 日本語をエスケープしない
            sort_keys=False,         # キー順維持(見やすさ重視)
            default_flow_style=False # ブロックスタイルで出力
        )

    # 文字列に出力
    s = yaml.safe_dump(data, allow_unicode=True, sort_keys=False)
    print(s)

if __name__ == "__main__":
    write_yaml()

# ini_examples.py
from pathlib import Path
from configparser import ConfigParser

BASE_DIR = Path(__file__).resolve().parent
INI_PATH = BASE_DIR / "config" / "config.ini"

def read_ini():
    config = ConfigParser()  # 既定の補間(BasicInterpolation)が有効
    config.read(INI_PATH, encoding="utf-8")

    print("sections:", config.sections())      # ['server'] など
    print("app_name:", config.get("DEFAULT", "app_name"))
    print("endpoint:", config.get("server", "endpoint"))  # %(host)s 等が展開される

if __name__ == "__main__":
    read_ini()

# ini_examples.py (続き)
def get_typed_values():
    config = ConfigParser()
    config.read(INI_PATH, encoding="utf-8")

    # 文字列/数値/真偽を型付きで取得
    app = config.get("DEFAULT", "app_name")          # str
    debug = config.getboolean("DEFAULT", "debug")    # bool(yes/true/on を True と解釈)
    retry_count = config.getint("DEFAULT", "retry_count")       # int
    retry_interval = config.getfloat("DEFAULT", "retry_interval")  # float
    use_tls = config.getboolean("server", "use_tls", fallback=False)

    print(app, debug, retry_count, retry_interval, use_tls)

if __name__ == "__main__":
    get_typed_values()

# ini_examples.py (続き)
def write_ini():
    config = ConfigParser()
    config.read(INI_PATH, encoding="utf-8")

    # 値を更新
    config.set("DEFAULT", "debug", "false")

    # 新しいセクションを追加
    if not config.has_section("paths"):
        config.add_section("paths")
    config.set("paths", "log_dir", "/var/log/sampleapp")
    config.set("paths", "data_dir", "/opt/sampleapp/data")

    out_path = BASE_DIR / "config" / "out_config.ini"
    with out_path.open("w", encoding="utf-8") as f:
        config.write(f)

    print("書き出しました:", out_path)

if __name__ == "__main__":
    write_ini()

[DEFAULT]
app_name = SampleApp
debug = false
retry_count = 3
retry_interval = 1.5

[server]
host = example.com
port = 8080
use_tls = yes
endpoint = https://%(host)s:%(port)s

[paths]
log_dir = /var/log/sampleapp
data_dir = /opt/sampleapp/data