单例类型提示的未解析引用[重复]

Question

有没有办法消除警告：

未解决的引用'DeviceManager'...

对于这种单例模式？

class DeviceManager:
    """ Holds all devices and manages their distributed use. """

    instance: Union[DeviceManager, None] = None  # warning Unresolved reference 'DeviceManager'

    @staticmethod
    def get_instance() -> DeviceManager:  # warning Unresolved reference 'DeviceManager'
        """ Singleton instance. """

        if DeviceManager.instance is None:
            DeviceManager()

        return cast(DeviceManager, DeviceManager.instance)

    def __init__(self) -> None:
        """ Create instance. """
        if DeviceManager.instance is None:
            DeviceManager.instance = self
        else:
            raise Exception("This class is a singleton!")

截图：

Answer 1

对于Python3.7的早期版本，还有一种方法。要避免出现警告，只需提供如下字符串类型：

class DeviceManager:
    """ Holds all devices and manages their distributed use. """

    instance: Union['DeviceManager', None] = None

    @staticmethod
    def get_instance() -> 'DeviceManager':
        """ Singleton instance. """

        if DeviceManager.instance is None:
            DeviceManager()

        return cast(DeviceManager, DeviceManager.instance)

    def __init__(self) -> None:
        """ Create instance. """
        if DeviceManager.instance is None:
            DeviceManager.instance = self
        else:
            raise Exception("This class is a singleton!")

资料来源：https://www.pythonsheets.com/notes/python-future.html

Answer 2

是的，如果您使用的是Python

问题是，在创建类时，它不存在，因此您的类型注释指向一些不存在的东西。

为了解决这个问题，您可以使用__future__导入注释中的，它将此类注释的计算推迟到类创建之后。

详见PEP 563。