Pythonの辞書やリストをJSON化 json.dumpとjson.loadを解説

# Pythonの辞書をJSON文字列に変換して表示する例
import json

data = {
    "title": "入門",
    "tags": ["python", "json"],
    "published": True,
    "count": 3
}

# ensure_ascii=False で日本語をそのまま出力、indent=2 で見やすく整形
json_str = json.dumps(data, ensure_ascii=False, indent=2)
print(json_str)

# JSONが数値・bool・nullをどう扱うかを確認
import json

data = {"ok": True, "nothing": None, "pi": 3.14}
s = json.dumps(data)        # 文字列化
print(s)                    # JSON文字列

obj = json.loads(s)         # Pythonオブジェクトへ戻す
print(type(obj["ok"]), obj["ok"])
print(type(obj["nothing"]), obj["nothing"])
print(type(obj["pi"]), obj["pi"])

# コメント付きJSONはエラーになる例
import json

json_text = """
{
  "name": "Taro", // ここはコメント(エラー)
  "age": 20
}
"""

try:
    json.loads(json_text)
except json.JSONDecodeError as e:
    print("JSONDecodeError:", e)

# users.json に辞書(リストを含む)を保存する
import json

users = {
    "users": [
        {"id": 1, "name": "山田太郎"},
        {"id": 2, "name": "Jane Doe"}
    ],
    "active": True
}

# UTF-8で書き込み、ensure_ascii=Falseで日本語をそのまま保存
with open("users.json", "w", encoding="utf-8") as f:
    json.dump(users, f, ensure_ascii=False, indent=2)

print("書き込み完了: users.json")

# users.json を読み込んで中身と型を確認
import json

with open("users.json", "r", encoding="utf-8") as f:
    loaded = json.load(f)

print(type(loaded))         # dictになる
print(loaded["users"][0])   # 最初のユーザー
print("active =", loaded["active"])

# ensure_ascii の違いを比較
import json

data = {"text": "こんにちは", "emoji": "🍣"}

s_default = json.dumps(data)  # 既定(ensure_ascii=True)
s_utf8 = json.dumps(data, ensure_ascii=False)

print("default:", s_default)
print("utf8   :", s_utf8)

# 整形(Pretty Print)と圧縮(Minify)の例
import json

data = {"z": 1, "a": [3, 2, 1]}

pretty = json.dumps(data, ensure_ascii=False, indent=2, sort_keys=True)
compact = json.dumps(data, ensure_ascii=False, separators=(",", ":"))

print("Pretty:\n", pretty)
print("Compact:", compact)

# UTF-8で書いてUTF-8で読む(文字化け対策の基本形)
import json

data = {"title": "文字化け対策", "note": "UTF-8で統一しましょう。"}

with open("utf8.json", "w", encoding="utf-8") as f:
    json.dump(data, f, ensure_ascii=False, indent=2)

with open("utf8.json", "r", encoding="utf-8") as f:
    loaded = json.load(f)

print("OK:", loaded)

# dumps で文字列化して型と中身を確認
import json

data = [{"id": 1}, {"id": 2}]
s = json.dumps(data, indent=2)
print(type(s), "\n", s)

# JSON文字列をPythonオブジェクトへ
import json

json_text = '{"a": 1, "flags": [true, false], "nothing": null}'
obj = json.loads(json_text)

print(type(obj), obj)               # dict
print(type(obj["flags"]), obj["flags"])  # list
print(type(obj["nothing"]), obj["nothing"])  # NoneType/None

# 4関数の最小例で違いを確認
import json

# ファイルに書く: dump
with open("tmp.json", "w", encoding="utf-8") as f:
    json.dump({"x": 1}, f)

# 文字列を作る: dumps
s = json.dumps({"x": 1})
print("dumps ->", type(s), s)

# ファイルから読む: load
with open("tmp.json", "r", encoding="utf-8") as f:
    obj1 = json.load(f)

# 文字列から読む: loads
obj2 = json.loads('{"x": 1}')

print("load  ->", type(obj1), obj1)
print("loads ->", type(obj2), obj2)

# 末尾カンマのエラー例
import json

bad = '{"a": 1,}'   # "1," の後ろのカンマが余計
try:
    json.loads(bad)
except json.JSONDecodeError as e:
    print("JSONDecodeError:", e)

# タプルは配列として保存され、読み戻すとリストになる
import json

t = (1, 2, 3)
s = json.dumps(t)       # JSON文字列へ
back = json.loads(s)    # Pythonへ戻す

print(type(s), s)       # str と "[1, 2, 3]"
print(type(back), back) # list と [1, 2, 3]

# 数値・bool・None の往復確認
import json

data = {"i": 1, "f": 2.5, "flag": False, "nothing": None}
s = json.dumps(data)
obj = json.loads(s)

print(type(obj["i"]).__name__, obj["i"])
print(type(obj["f"]).__name__, obj["f"])
print(type(obj["flag"]).__name__, obj["flag"])
print(type(obj["nothing"]).__name__, obj["nothing"])