如何让Python ABC抽象方法仅约束子类而允许基类实例化？

要实现「基类可实例化、子类必须强制实现指定属性」的需求，无需额外中间类违反DRY原则，有几种灵活的实现方式：

方案一：自定义子类检查装饰器 #

写一个装饰器标记基类，在子类继承时自动检查是否实现目标属性，逻辑清晰且代码侵入性低：

def enforce_subclass_implementation(attr_name):
    def decorator(base_cls):
        original_subclass = base_cls.__subclasshook__
        def new_subclasshook(subclass):
            # 保留原有子类检查逻辑（如果存在）
            result = original_subclass(subclass) if original_subclass else NotImplemented
            if result is NotImplemented or result:
                # 检查子类是否重写了目标属性
                if not hasattr(subclass, attr_name) or getattr(subclass, attr_name) is getattr(base_cls, attr_name):
                    raise TypeError(f"子类 {subclass.__name__} 必须实现属性 {attr_name}")
            return result
        base_cls.__subclasshook__ = new_subclasshook
        return base_cls
    return decorator

# 用装饰器标记基类
@enforce_subclass_implementation("similar_entities")
class Entity:
    @property
    def similar_entities(self) -> list[str]:
        return []

# 基类可正常实例化
Entity()  # 无报错

class SubEntity1(Entity):
    @property
    def similar_entities(self):
        return ["a", "b", "c"]

SubEntity1()  # 可实例化

class SubEntity2(Entity):
    pass

SubEntity2()  # 触发TypeError: 子类 SubEntity2 必须实现属性 similar_entities

方案二：自定义元类 #

利用元类的底层特性，在子类创建时直接检查是否实现指定属性，适合复杂类契约场景：

class EntityMeta(type):
    def __new__(cls, name, bases, attrs):
        new_cls = super().__new__(cls, name, bases, attrs)
        # 仅对继承自Entity的子类做检查（排除基类自身）
        if bases != () and "similar_entities" not in attrs:
            for base in bases:
                if base.__name__ == "Entity":
                    raise TypeError(f"子类 {name} 必须实现属性 similar_entities")
        return new_cls

class Entity(metaclass=EntityMeta):
    @property
    def similar_entities(self) -> list[str]:
        return []

# 基类可正常实例化
Entity()  # 无报错

class SubEntity1(Entity):
    @property
    def similar_entities(self):
        return ["a", "b", "c"]

SubEntity1()  # 可实例化

class SubEntity2(Entity):
    pass

SubEntity2()  # 触发TypeError: 子类 SubEntity2 必须实现属性 similar_entities

方案三：利用__init_subclass__钩子（轻量方案） #

借助Python内置的子类初始化钩子，在子类定义时自动执行检查，无需依赖额外工具：

class Entity:
    @property
    def similar_entities(self) -> list[str]:
        return []
    
    def __init_subclass__(cls):
        # 排除基类自身，检查子类是否重写属性
        if cls is not Entity and (not hasattr(cls, "similar_entities") or getattr(cls, "similar_entities") is Entity.similar_entities):
            raise TypeError(f"子类 {cls.__name__} 必须实现属性 similar_entities")

# 基类可正常实例化
Entity()  # 无报错

class SubEntity1(Entity):
    @property
    def similar_entities(self):
        return ["a", "b", "c"]

SubEntity1()  # 可实例化

class SubEntity2(Entity):
    pass

# 定义子类时直接触发错误，无需等到实例化
# TypeError: 子类 SubEntity2 必须实现属性 similar_entities

内容的提问来源于stack exchange，提问作者RedRuin