エラーと例外処理

def divide(a: int, b: int) -> float:
    try:
        return a / b
    except ZeroDivisionError:
        return float("inf")

print(divide(10, 2))   # 5.0
print(divide(10, 0))   # inf

if b != 0:
    result = a / b
else:
    result = float("inf")

try:
    result = a / b
except ZeroDivisionError:
    result = float("inf")

def parse_int(value: str) -> int | None:
    try:
        return int(value)
    except (ValueError, TypeError):
        return None

print(parse_int("42"))   # 42
print(parse_int("abc"))  # None
print(parse_int(None))   # None

try:
    do_work()
except ValueError as e:
    print(f"不正な値: {e}")
except KeyError as e:
    print(f"キーがない: {e}")
except Exception as e:
    print(f"それ以外: {e}")
    raise   # 再送出

def read_file(path: str) -> str:
    f = None
    try:
        f = open(path)
        data = f.read()
    except FileNotFoundError:
        return ""
    else:
        # 例外なしで try が終わったときだけ実行
        print(f"読み取り成功: {len(data)} bytes")
        return data
    finally:
        # 例外の有無に関わらず常に実行
        if f is not None:
            f.close()

def read_file(path: str) -> str:
    try:
        with open(path) as f:
            return f.read()
    except FileNotFoundError:
        return ""

def withdraw(balance: int, amount: int) -> int:
    if amount <= 0:
        raise ValueError("出金額は正の数でなければなりません")
    if amount > balance:
        raise ValueError(f"残高不足: {balance} < {amount}")
    return balance - amount

try:
    do_work()
except KeyError:
    log("キーがない発生")
    raise   # 引数なしで raise — 捕まえた例外をそのまま再送出

def load_config(path: str) -> dict:
    try:
        with open(path) as f:
            return parse(f.read())
    except (OSError, ValueError) as e:
        raise ConfigError(f"設定の読み込み失敗: {path}") from e

BaseException
 ├─ SystemExit          ← sys.exit()
 ├─ KeyboardInterrupt   ← Ctrl+C
 └─ Exception           ← 通常はここから下だけ捕まえる
     ├─ ValueError
     │   └─ UnicodeError
     ├─ TypeError
     ├─ LookupError
     │   ├─ KeyError
     │   └─ IndexError
     ├─ ArithmeticError
     │   └─ ZeroDivisionError
     ├─ OSError
     │   └─ FileNotFoundError
     └─ ...

try:
    risky()
except:           # bare except — KeyboardInterrupt も捕まる
    pass

# あるいは
except BaseException:   # 同じ問題
    pass

try:
    risky()
except Exception as e:
    log(e)

class AppError(Exception):
    """このアプリのすべての例外のベース。"""

class ConfigError(AppError):
    """設定関連エラー。"""

class DatabaseError(AppError):
    """DB 関連エラー。"""

class RowNotFoundError(DatabaseError):
    """必要な行がない。"""
    def __init__(self, table: str, key: str):
        super().__init__(f"{table} で {key} が見つかりません")
        self.table = table
        self.key = key

try:
    user = repo.get_user("u1")
except RowNotFoundError as e:
    return Response(status=404)
except DatabaseError as e:
    log_critical(e)
    return Response(status=500)

def f() -> int:
    try:
        return 1
    finally:
        return 2   # こちらが勝つ

print(f())   # 2

errors = ExceptionGroup(
    "複数の作業が失敗",
    [ValueError("a"), TypeError("b"), ValueError("c")],
)
raise errors

try:
    raise ExceptionGroup(
        "複数の失敗",
        [ValueError("a"), TypeError("b"), ValueError("c")],
    )
except* ValueError as eg:
    print("値エラーたち:", [str(e) for e in eg.exceptions])
except* TypeError as eg:
    print("型エラーたち:", [str(e) for e in eg.exceptions])

def calc_average(items: list[int]) -> float:
    assert len(items) > 0, "空リストでは平均を求められない"
    return sum(items) / len(items)

def serve(user: User | None):
    # ✅ システム前提: 認証ミドルウェアが None を防いだはず
    assert user is not None, "auth middleware が抜けている?"

    # ❌ ユーザー入力を assert で防ぐのは NG
    # assert age >= 0, "負の年齢は不可"  ← -O で消える
    if age < 0:
        raise ValueError("年齢は 0 以上")

try:
    process(item)
except Exception:
    pass    # すべての例外を無視 — バグを隠す

try:
    process(item)
except ValueError as e:
    log(f"不正な項目: {e}")

try:
    a = compute()
    b = transform(a)
    c = validate(b)
    d = save(c)
    return d
except SomeError:
    ...