如何设置 class 方法 return 从 class 属性动态输入

Question

我正在做我的一个宠物项目，发现了这个小问题。我想在基础 class 中使用类型，问题是 class 方法的 return 类型是由 subclasses 设置的字段定义的。这是我的基地 class

class BaseRepository(metaclass=RequiredAttributes('model')):

    model = None  # The subclasses have to define the this field with a class type.
                  # All the class methods will return objects of the same
                  # type as this field.

    def get(self, id) -> ??:
        # return an object of the type defined in mode
        ...


class UserRepo(BaseRepository): # subclass

    model = User # class type

    ...

我想将 get 函数的类型设置为与模型字段中定义的对象类型相同。

关于我怎样才能完成这样的事情有什么建议吗？

Answer 1

如果没有元类，这个 passes MyPy, which is correctly able to infer that x is of type User. I wasn't able to test it with the metaclass, as I don't have your metaclass definition. The solution uses a parameterised Generic as a base class, which allows you to then assert to the type-checker that different methods across the class will have the same parameter and return argument types. A similar technique is used in Django's stub files。

N.B。你必须从 typing 模块导入 Type 并且如果你使用 Python <= 3.8.

from typing import Optional, Generic, TypeVar


M = TypeVar('M')


class BaseRepository(Generic[M], metaclass=RequiredAttributes('model')):
    model: Optional[type[M]] = None     # The subclasses have to define the this field with a class type.
                                        # All the class methods will return objects of the same
                                        # type as this field.

    def get(self, id: int) -> M: ...
        # return an object of the type defined in mode
        

class User:
    pass


class UserRepo(BaseRepository[User]): # subclass
    model = User # class type
    

x = UserRepo().get(1)
reveal_type(x)  # Revealed type is "__main__.User*"

如何设置 class 方法 return 从 class 属性动态输入

How to set a class method return typing dynamically from a class property

python

type-hinting

python-3.x

python-typing

如何设置 class 方法 return 从 class 属性 动态输入

How to set a class method return typing dynamically from a class property

python

type-hinting

python-3.x

python-typing

如何设置 class 方法 return 从 class 属性动态输入