Python中的描述符协议与数据验证实现

描述符协议是Python中实现属性访问控制的重要机制，今天我们将重点学习如何利用描述符实现数据验证功能。

1. 描述符协议回顾

描述符是一个包含__get__、__set__或__delete__方法的类，分为数据描述符和非数据描述符：

• 数据描述符：定义了__set__或__delete__方法
• 非数据描述符：只定义了__get__方法

2. 基础数据验证描述符实现

让我们从最简单的数值范围验证开始：

class RangeValidator:
    def __init__(self, min_value, max_value):
        self.min_value = min_value
        self.max_value = max_value
        self.storage_name = None
    
    def __set_name__(self, owner, name):
        # 自动获取属性名称
        self.storage_name = name
    
    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__.get(self.storage_name)
    
    def __set__(self, instance, value):
        if not (self.min_value <= value <= self.max_value):
            raise ValueError(f"值必须在{self.min_value}和{self.max_value}之间")
        instance.__dict__[self.storage_name] = value

3. 类型验证增强

添加类型检查确保数据类型的正确性：

class TypedRangeValidator:
    def __init__(self, value_type, min_value, max_value):
        self.value_type = value_type
        self.min_value = min_value
        self.max_value = max_value
        self.storage_name = None
    
    def __set_name__(self, owner, name):
        self.storage_name = name
    
    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__.get(self.storage_name)
    
    def __set__(self, instance, value):
        # 类型检查
        if not isinstance(value, self.value_type):
            raise TypeError(f"期望类型{self.value_type}，但收到{type(value)}")
        
        # 范围检查
        if not (self.min_value <= value <= self.max_value):
            raise ValueError(f"值必须在{self.min_value}和{self.max_value}之间")
        
        instance.__dict__[self.storage_name] = value

4. 字符串验证描述符

针对字符串的特殊验证需求：

class StringValidator:
    def __init__(self, min_length=0, max_length=None, pattern=None):
        self.min_length = min_length
        self.max_length = max_length
        self.pattern = pattern
        self.storage_name = None
    
    def __set_name__(self, owner, name):
        self.storage_name = name
    
    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__.get(self.storage_name)
    
    def __set__(self, instance, value):
        if not isinstance(value, str):
            raise TypeError("值必须是字符串")
        
        if len(value) < self.min_length:
            raise ValueError(f"字符串长度不能小于{self.min_length}")
        
        if self.max_length and len(value) > self.max_length:
            raise ValueError(f"字符串长度不能大于{self.max_length}")
        
        if self.pattern and not self.pattern.match(value):
            raise ValueError("字符串格式不符合要求")
        
        instance.__dict__[self.storage_name] = value

5. 完整的应用示例

将验证描述符应用到实际类中：

import re

class Person:
    # 年龄验证：整数，0-150之间
    age = TypedRangeValidator(int, 0, 150)
    
    # 姓名验证：字符串，1-50字符，只包含字母
    name = StringValidator(min_length=1, max_length=50, 
                          pattern=re.compile(r'^[a-zA-Z\s]+$'))
    
    # 邮箱验证
    email = StringValidator(min_length=5, max_length=100,
                           pattern=re.compile(r'^[^@]+@[^@]+\.[^@]+$'))
    
    def __init__(self, name, age, email):
        self.name = name
        self.age = age
        self.email = email

# 测试验证功能
try:
    person = Person("Alice", 25, "alice@example.com")  # 有效数据
    print("创建成功")
    
    person.age = 200  # 触发范围验证错误
except ValueError as e:
    print(f"验证错误: {e}")

try:
    person.name = "Alice123"  # 触发格式验证错误
except ValueError as e:
    print(f"验证错误: {e}")

6. 高级特性 - 延迟验证

某些场景下需要支持延迟验证或批量设置：

class LazyValidator:
    def __init__(self, validator):
        self.validator = validator
        self.storage_name = None
        self.pending_value = None
    
    def __set_name__(self, owner, name):
        self.storage_name = name
    
    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__.get(self.storage_name)
    
    def __set__(self, instance, value):
        # 存储值但不立即验证
        self.pending_value = value
    
    def validate(self, instance):
        """手动触发验证"""
        if self.pending_value is not None:
            # 使用原始验证器进行验证
            self.validator.__set__(instance, self.pending_value)
            self.pending_value = None

7. 验证描述符的最佳实践

1. 明确的错误信息：提供具体、可操作的错误信息
2. 性能考虑：避免在__get__中进行复杂验证
3. 继承支持：确保描述符在继承体系中正常工作
4. 文档化：为每个验证器提供清晰的文档说明

这种基于描述符的数据验证方法提供了类型安全、可重用和声明式的验证机制，是构建健壮Python应用的重要工具。