Python assert断言详解 - assert语法与if raise区别

# assert的两种语法形式
assert condition                    # 只检查条件
assert condition, error_message     # 条件为假时显示错误信息

# 等效于
if __debug__:
    if not condition:
        raise AssertionError(error_message)

# 前置条件检查
def calculate_discount(price, discount_rate):
    assert isinstance(price, (int, float)), "价格必须是数字"
    assert price >= 0, "价格不能为负数"
    assert 0 <= discount_rate <= 1, "折扣率必须在0到1之间"
    return price * (1 - discount_rate)

# 不变量检查
class BankAccount:
    def __init__(self, balance=0):
        self.balance = balance
    
    def withdraw(self, amount):
        assert amount > 0, "取款金额必须为正数"
        old_balance = self.balance
        self.balance -= amount
        # 后置条件：余额应该减少
        assert self.balance == old_balance - amount, "余额计算错误"
        assert self.balance >= 0, "余额不能为负"
        return self.balance

# assert：用于验证开发者自己的假设
def process_list(items):
    assert isinstance(items, list), "内部错误：应该传入列表"
    # 这是代码逻辑假设，不应该被用户触发
    return sorted(items)

# if raise：用于验证用户输入或外部数据
def save_user_data(data):
    if not isinstance(data, dict):
        raise TypeError("用户数据必须是字典格式")
    if "name" not in data:
        raise ValueError("用户数据必须包含name字段")
    # 这是对外部输入的验证，必须在生产环境中生效
    return save_to_db(data)

# 验证优化模式的影响
def test_assert():
    print(f"__debug__ = {__debug__}")
    assert False, "这个断言在-O模式下不会执行"
    print("assert后面的代码执行了")

# 正常模式运行：python script.py
# 输出：AssertionError: 这个断言在-O模式下不会执行

# 优化模式运行：python -O script.py
# 输出：__debug__ = False
#       assert后面的代码执行了

def binary_search(arr, target):
    """二分查找，使用断言验证前提条件"""
    assert len(arr) > 0, "数组不能为空"
    # 二分查找要求数组已排序
    assert all(arr[i] <= arr[i+1] for i in range(len(arr)-1)), \
        "数组必须是有序的"
    
    left, right = 0, len(arr) - 1
    while left <= right:
        mid = (left + right) // 2
        if arr[mid] == target:
            return mid
        elif arr[mid] < target:
            left = mid + 1
        else:
            right = mid - 1
    
    # 未找到时的不变量
    assert left > right, "循环结束时left应该大于right"
    return -1

# 测试
assert binary_search([1, 3, 5, 7, 9], 5) == 2
assert binary_search([1, 3, 5, 7, 9], 4) == -1
print("所有测试通过！")

class Shape:
    def area(self):
        raise NotImplementedError

class Circle(Shape):
    def __init__(self, radius):
        assert radius > 0, "半径必须大于0"
        self.radius = radius
    
    def area(self):
        return 3.14159 * self.radius ** 2

class Rectangle(Shape):
    def __init__(self, width, height):
        assert width > 0 and height > 0, "宽高必须大于0"
        self.width = width
        self.height = height
    
    def area(self):
        return self.width * self.height

def create_shape(shape_type, **kwargs):
    """工厂函数"""
    if shape_type == "circle":
        return Circle(kwargs["radius"])
    elif shape_type == "rectangle":
        return Rectangle(kwargs["width"], kwargs["height"])
    else:
        assert False, f"未知的形状类型：{shape_type}"

# 测试
shapes = [
    create_shape("circle", radius=5),
    create_shape("rectangle", width=3, height=4),
]

for shape in shapes:
    area = shape.area()
    assert area > 0, f"面积必须为正数，实际：{area}"
    print(f"{shape.__class__.__name__}面积：{area:.2f}")