Python中的描述符（Descriptor）高级应用与实战

描述符是Python中一个强大的特性，它允许对象自定义属性访问的行为。你已经了解了描述符的基本概念，现在我们来深入探讨其高级应用和实际使用场景。

1. 描述符协议回顾
描述符是实现了特定协议（__get__、__set__、__delete__）的类。根据实现的协议方法不同，分为：

• 数据描述符：实现__set__或__delete__
• 非数据描述符：只实现__get__

2. 描述符的优先级规则
当实例属性、类属性和描述符同名时，访问优先级为：

1. 数据描述符（最高优先级）
2. 实例属性
3. 非数据描述符
4. 类属性（最低优先级）

class DataDescriptor:
    def __get__(self, instance, owner):
        return "数据描述符"
    
    def __set__(self, instance, value):
        pass

class NonDataDescriptor:
    def __get__(self, instance, owner):
        return "非数据描述符"

class Test:
    data_desc = DataDescriptor()
    non_data_desc = NonDataDescriptor()

t = Test()
t.data_desc = "实例属性"  # 数据描述符优先，这行实际上调用描述符的__set__
t.non_data_desc = "实例属性"  # 非数据描述符，创建实例属性
print(t.data_desc)    # 输出：数据描述符
print(t.non_data_desc) # 输出：实例属性

3. 延迟计算属性（Lazy Property）
使用描述符实现延迟初始化，只有在第一次访问时才进行计算：

class LazyProperty:
    def __init__(self, method):
        self.method = method
        self.method_name = method.__name__
    
    def __get__(self, instance, owner):
        if instance is None:
            return self
        # 第一次访问时计算结果并缓存
        value = self.method(instance)
        setattr(instance, self.method_name, value)  # 替换描述符为计算结果
        return value

class HeavyComputation:
    @LazyProperty
    def expensive_result(self):
        print("执行复杂计算...")
        return sum(i*i for i in range(10**6))

obj = HeavyComputation()
print("第一次访问:")
print(obj.expensive_result)  # 会执行计算
print("第二次访问:")
print(obj.expensive_result)  # 直接返回缓存结果

4. 验证型描述符
确保属性值满足特定条件：

class Validated:
    def __init__(self, name=None, min_value=None, max_value=None):
        self.name = name
        self.min_value = min_value
        self.max_value = max_value
    
    def __set_name__(self, owner, name):
        self.name = name
    
    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__.get(self.name)
    
    def __set__(self, instance, value):
        if self.min_value is not None and value < self.min_value:
            raise ValueError(f"{self.name}不能小于{self.min_value}")
        if self.max_value is not None and value > self.max_value:
            raise ValueError(f"{self.name}不能大于{self.max_value}")
        instance.__dict__[self.name] = value

class Person:
    age = Validated(min_value=0, max_value=150)
    height = Validated(min_value=0)
    
    def __init__(self, age, height):
        self.age = age
        self.height = height

try:
    p = Person(200, 180)  # 会抛出异常
except ValueError as e:
    print(f"错误: {e}")

5. 观察者模式描述符
当属性变化时自动通知观察者：

class Observable:
    def __init__(self):
        self.observers = []
    
    def __set_name__(self, owner, name):
        self.name = name
    
    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__.get(self.name)
    
    def __set__(self, instance, value):
        old_value = instance.__dict__.get(self.name)
        instance.__dict__[self.name] = value
        if old_value != value:
            self.notify(instance, old_value, value)
    
    def add_observer(self, observer):
        self.observers.append(observer)
    
    def notify(self, instance, old_value, new_value):
        for observer in self.observers:
            observer(instance, self.name, old_value, new_value)

class Stock:
    price = Observable()
    
    def __init__(self, symbol, price):
        self.symbol = symbol
        self.price = price

def price_change_handler(instance, attr_name, old_value, new_value):
    print(f"{instance.symbol}价格从{old_value}变为{new_value}")

stock = Stock("AAPL", 100)
Stock.price.add_observer(price_change_handler)
stock.price = 105  # 会自动触发通知

6. 描述符在框架中的应用
许多流行框架都使用描述符，比如Django的模型字段：

# 简化的Django风格字段描述符
class CharField:
    def __init__(self, max_length=255):
        self.max_length = max_length
    
    def __set_name__(self, owner, name):
        self.name = name
    
    def __get__(self, instance, owner):
        return instance.__dict__.get(self.name)
    
    def __set__(self, instance, value):
        if not isinstance(value, str):
            raise TypeError("必须是字符串")
        if len(value) > self.max_length:
            raise ValueError(f"长度不能超过{self.max_length}")
        instance.__dict__[self.name] = value

class User:
    username = CharField(max_length=50)
    email = CharField(max_length=100)
    
    def __init__(self, username, email):
        self.username = username
        self.email = email

7. 描述符的最佳实践

1. 使用__set_name__自动获取属性名（Python 3.6+）
2. 在__get__中正确处理instance is None的情况
3. 将数据存储在实例的__dict__中避免递归
4. 考虑描述符的可继承性

描述符是Python元编程的核心工具之一，合理使用可以让代码更加优雅和强大。