通用单例的类型提示？

Question

我有一个基础服务，它提供了所有服务的通用方法。此外，此 BaseService 用作服务注册表：

class BaseService:
    instances = {}

    @classmethod
    def get_instance(cls) -> 'BaseService':
        if cls.instances.get(cls) is None:
            cls.instances[cls] = cls()
        return cls.instances[cls]

class Service1(BaseService):
    pass

class Service2(BaseService):
    pass

Service1.get_instance()
Service2.get_instance()
Service1.get_instance()

get_instance() 方法正在返回子 class 实例，我觉得当前注释 -> 'BaseService' 不正确。我应该如何正确注释此方法？

Answer 1

正如我在评论中所说，为基数class的class方法执行此操作是有问题，因为根据定义，该方法将与任何 subclasses 共享。对于单身人士来说尤其如此。

解决方法是为每个子class 提供具有适当return 值注释的相似命名方法。虽然这可以使用 class 装饰器来完成，如我的答案的早期版本所示，但使用 metaclass 似乎是一种更简洁的方法，因此我相应地更新了我的答案：

class BaseServiceMeta(type):
    """ Metaclass that properly annotates the return value of the get_instance() method of
        any subclasses of the BaseService class.
    """
    def __new__(metaclass, classname, bases, classdict):
        cls = super(metaclass, metaclass).__new__(metaclass, classname, bases, classdict)
        if classname != 'BaseService':  # subclass?

            # define function with the correct return value annotation
            def get_instance() -> classname:
                return super(cls, cls).get_instance()  # call superclass classmethod

            setattr(cls, 'get_instance', get_instance)  # override inherited method

        return cls

class BaseService(metaclass=BaseServiceMeta):  # metaclass added
    instances = {}

    @classmethod
    def get_instance(cls) -> 'BaseService':
        if cls.instances.get(cls) is None:
            cls.instances[cls] = cls()
        return cls.instances[cls]

class Service1(BaseService):
    pass

class Service2(BaseService):
    pass

# show that the methods have the correct return annotation
print(repr(BaseService.get_instance.__annotations__['return']))  # -> 'BaseService'
print(repr(   Service1.get_instance.__annotations__['return']))  # -> 'Service1'
print(repr(   Service2.get_instance.__annotations__['return']))  # -> 'Service2'

# call subclass methods to show they return the correct singleton instance of each type
print(Service1.get_instance())  # -> <__main__.Service1 object at 0x004A07D0>
print(Service2.get_instance())  # -> <__main__.Service2 object at 0x004A07F0>
print(Service1.get_instance())  # -> <__main__.Service1 object at 0x004A07D0>

通用单例的类型提示？

Type hinting for generic singleton?

python

generics

singleton

types

hint