例外の発生と再発生

はじめに

組み込み例外の raise

def divide(a, b):
    if b == 0:
        raise ValueError('Denominator cannot be zero')
    return a / b
try:
    divide(5, 0)
except ValueError as e:
    print(e)

同じ例外の再送出

def process(data):
    try:
        return int(data)
    except ValueError:
        print('logging error')
        raise  # re-raises original
try:
    process('abc')
except ValueError:
    print('caller caught it')

from による例外の連鎖

try:
    int('abc')
except ValueError as original:
    raise RuntimeError('processing failed') from original

例外の連鎖を抑制する

try:
    int('abc')
except ValueError:
    raise RuntimeError('clean error') from None

assert 文

def sqrt(x):
    assert x >= 0, f'Input must be non-negative, got {x}'
    return x ** 0.5
try:
    sqrt(-1)
except AssertionError as e:
    print(e)

条件付きの raise

def process(items):
    if not items:
        raise ValueError('items must not be empty')
    return items[0]
print(process([1, 2, 3]))

再送出と新しい例外の違い

def outer():
    try:
        inner()
    except ValueError as e:
        raise RuntimeError('outer failed') from e
def inner():
    raise ValueError('inner failed')
try:
    outer()
except RuntimeError as e:
    print(e, e.__cause__)

finally 内の例外

try:
    try:
        raise ValueError('original')
    finally:
        print('finally ran')
except ValueError as e:
    print('caught:', e)

warnings.warn と raise の違い

import warnings
def old_func():
    warnings.warn('Deprecated', DeprecationWarning)
    return 42
print(old_func())

raise from None パターン

class DBError(Exception): pass
def query(q):
    try:
        raise ConnectionError('refused')
    except ConnectionError:
        raise DBError('query failed') from None
try:
    query('SELECT 1')
except DBError as e:
    print(e)

クイックチェック

まとめ

次へ進みましょう