Python OOP测试未通过求助：page_count属性赋值校验失效

问题详情 #

写了个Python的Book类，本地运行看似正常，但过不了单元测试。测试要求给page_count赋非整数值时输出指定字符串，结果测试断言显示未捕获到预期输出。

我的代码 #

class Book:
    def __init__(self, title, page_count):
        self.title = title
        if isinstance(page_count, int):
            self.page_count = page_count
        else:
            print("page_count must be an integer")

测试失败信息 #

def test_requires_int_page_count(self):
    '''prints "page_count must be an integer" if page_count is not an integer.'''
    book = Book("And Then There Were None", 272)
    captured_out = io.StringIO()
    sys.stdout = captured_out
    book.page_count = "not an integer"
    sys.stdout = sys.__stdout__
>   assert captured_out.getvalue() == "page_count must be an integer\n"
E   AssertionError: assert '' == 'page_count m... an integer\n'
E     - page_count must be an integer

问题根因 #

你的代码只在创建对象初始化阶段做了page_count的类型检查和打印，但测试是在对象创建完成后直接给book.page_count赋值，这时候走的是Python默认的属性赋值逻辑，根本没触发你写的检查代码，所以没有输出。

修复方案 #

用Python的@property装饰器给page_count添加setter方法，这样无论是初始化还是后续赋值，都会经过类型检查：

class Book:
    def __init__(self, title, page_count):
        self.title = title
        self._page_count = None
        # 调用setter完成初始化，确保检查逻辑生效
        self.page_count = page_count

    @property
    def page_count(self):
        return self._page_count

    @page_count.setter
    def page_count(self, value):
        if isinstance(value, int):
            self._page_count = value
        else:
            print("page_count must be an integer")

关键说明 #

• 用_page_count作为私有存储变量，避免和@property装饰的page_count名称冲突
• 初始化时通过self.page_count = page_count调用setter，保证初始化阶段的类型检查也能生效
• 后续给book.page_count赋值时，会自动触发setter里的逻辑，满足测试的输出要求

内容的提问来源于stack exchange，提问作者Levin Mwanganyi