AssertionError

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x = 5
assert x == 10  # AssertionError

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items = []
assert items  # AssertionError（空リストはFalsy）

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value = "hello"
assert isinstance(value, int)  # AssertionError

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x = 5
assert x == 10, f"Expected 10, got {x}"
# AssertionError: Expected 10, got 5

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# assertは最適化モード（python -O）で無視される

# NG: 本番コードでassert
def withdraw(amount):
    assert amount > 0  # -O で無視される

# OK: 明示的なチェック
def withdraw(amount):
    if amount <= 0:
        raise ValueError("amount must be positive")

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# pytest はassertを拡張して詳細なエラーを表示
def test_addition():
    assert 1 + 1 == 3
    # AssertionError: assert 2 == 3
    #  +  where 2 = 1 + 1

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class ValidationError(Exception):
    pass

def validate_age(age):
    if not isinstance(age, int):
        raise ValidationError("age must be an integer")
    if age < 0:
        raise ValidationError("age must be non-negative")

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def greet(name: str) -> str:
    return f"Hello, {name}"

# mypyが型エラーを検出
greet(123)  # error: Argument 1 has incompatible type "int"; expected "str"

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# 1. 内部の不変条件チェック
def binary_search(arr, target):
    assert arr == sorted(arr), "Array must be sorted"
    # ...

# 2. 開発中のデバッグ
def process(data):
    result = transform(data)
    assert result is not None  # 開発中のみ
    return result

# 3. テストコード
def test_user_creation():
    user = create_user("John")
    assert user.name == "John"
    assert user.id is not None