Python 加入类型注解后，父类到子类的强制转换怎么处理？

[lanlanye]

先贴代码：

class AbstractMapper(ABC):

    def __init__(self):
        self._loaded_map: Dict["ObjectId", "DomainObject"] = {}

    def _abstract_find(self, id_: "ObjectId") -> "DomainObject":
        stmt = self.find_statement()
        try:
            rs = cursor.execute(stmt, (id_,)).fetchone()
            res = self._load(rs)
            return res
        except Exception as e:
            ...
        finally:
            ...

    def _load(self, rs: tuple) -> "DomainObject":
        if rs[0] in self._loaded_map:
            return self._loaded_map[rs[0]]
        res = self._do_load(rs)
        self._loaded_map[res.id] = res
        return res

    @abstractmethod
    def _do_load(self, rs: tuple) -> "DomainObject":
        ...

    @abstractmethod
    def find_statement(self) -> str:
        ...


class ArtistMapper(AbstractMapper):

    def find_statement(self) -> str:
        return "select id, name from artists where id = $1 limit 1"

    def _do_load(self, rs: tuple) -> "DomainObject":
        id_ = rs[0]
        name = rs[1]
        return Artist(id_, name)

    def find(self, id_) -> "Artist":
        return self._abstract_find(id_)  # Expression of type "DomainObject" cannot be assigned to return type "Artist"   "DomainObject" is incompatible with "Artist"

上面的代码中，_abstract_find() 的返回值标注为 DomainObject 类型，而在 ArtistMapper 中 find 方法需要返回一个 Artist 实例，Artist 是 DomainObject 的子类。

我看到 Java 代码中这里是通过强制类型转换解决的，Python 中这样的代码虽然可以正确运行，但无法通过静态类型检查，想知道有什么办法可以通过检查？

[shyrock]

Artist 跟 DomainObject 什么关系？

[lanlanye]

@shyrock Artist 继承自 DomainObject

[heqing]

使用 Generic

[hsfzxjy]

粗暴点就用 typing.cast ，规范点就给 AbstractMapper 加范型参数

[t2jk4000]

用 Self [https://peps.python.org/pep-0673/]( https://peps.python.org/pep-0673/)