Python属性(Property)优雅掌控对象数据的完全指南

class Wallet:
    def __init__(self, balance):
        self.balance = balance

my_wallet = Wallet(100)
print(my_wallet.balance)  # 输出: 100
my_wallet.balance = -50   # 这里的逻辑显然有问题，但代码运行毫无报错！

class Wallet:
    def __init__(self, balance):
        self._balance = balance  # 使用下划线表示“受保护”的内部变量

    def get_balance(self):
        return self._balance

    def set_balance(self, value):
        if value < 0:
            print("错误：余额不能为负数！")
        else:
            self._balance = value

w = Wallet(100)
w.set_balance(-50)  # 输出: 错误：余额不能为负数！

class Wallet:
    def __init__(self, balance):
        self._balance = balance

    @property
    def balance(self):
        """这是一个只读属性"""
        return self._balance

w = Wallet(100)
print(w.balance)  # 看起来像是在访问属性，实际上执行了 balance() 方法
# w.balance = 200  # 报错: AttributeError: can't set attribute

class Wallet:
    def __init__(self, balance):
        self._balance = balance

    @property
    def balance(self):
        return self._balance

    @balance.setter
    def balance(self, value):
        if value < 0:
            raise ValueError("余额不能为负数")
        self._balance = value

    @balance.deleter
    def balance(self):
        print("钱包已销毁")
        del self._balance

w = Wallet(100)
w.balance = 200  # 调用 setter
print(w.balance) # 200

try:
    w.balance = -50
except ValueError as e:
    print(e)  # 输出: 余额不能为负数

del w.balance   # 调用 deleter

class HeavyDataProcessor:
    def __init__(self, data):
        self.data = data
        self._result = None  # 缓存计算结果

    @property
    def result(self):
        if self._result is None:
            print("正在执行昂贵的计算...")
            # 模拟耗时操作
            self._result = sum(self.data) * len(self.data)
        return self._result

processor = HeavyDataProcessor([1, 2, 3, 4, 5])
# 此时还没有进行计算
print("准备获取结果")
val = processor.result  # 第一次访问，触发计算
print(val)
val2 = processor.result # 第二次访问，直接返回缓存，不再计算

class Person:
    def __init__(self, birth_year):
        self._birth_year = birth_year

    @property
    def age(self):
        # 外部依然访问 .age，但内部逻辑已经改变
        import datetime
        return datetime.datetime.now().year - self._birth_year
    
    @age.setter
    def age(self, value):
        # 通过设置年龄反向推算出生年份
        import datetime
        self._birth_year = datetime.datetime.now().year - value

p = Person(1990)
print(p.age)  # 外部调用者完全不知道内部逻辑变了
p.age = 30
print(p._birth_year) # 内部状态已更新

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    @property
    def area(self):
        return self.width * self.height

    @property
    def perimeter(self):
        return 2 * (self.width + self.height)

rect = Rectangle(10, 5)
# area 和 perimeter 看起来就像普通的属性，但实际上是由 width 和 height 动态计算的
print(f"面积: {rect.area}, 周长: {rect.perimeter}")