Mypy无法识别单例类属性，抛出[attr-defined]和[no-untyped-def]错误

原代码 #

class WaitService:
    _instance = None

    def __new__(cls, name: str = "Default"):
        if not cls._instance:
            cls._instance = super(WaitService, cls).__new__(cls)
            cls._instance.name = name
        return cls._instance


if __name__ == '__main__':
    w = WaitService("at till 9pm")
    print(w.name)

Mypy校验错误 #

error: "WaitService" has no attribute "name" [attr-defined]
error: Function is missing a return type annotation [no-untyped-def]

错误解决方法 #

1. 修复"has no attribute 'name'"错误

Mypy无法识别动态添加的name属性，需要在类中显式声明该属性的类型，让类型检查器知晓其存在：

• 在类内部添加name: str的类型注解。

2. 修复"missing a return type annotation"错误

__new__方法需要明确标注返回类型，由于是单例模式，返回的是当前类的实例，直接标注返回类型为WaitService（Python 3.10+可使用更简洁的Self类型）。

修正后的代码（兼容Python 3.8+） #

from typing import Optional

class WaitService:
    _instance: Optional['WaitService'] = None
    name: str  # 显式声明name属性类型

    def __new__(cls, name: str = "Default") -> 'WaitService':
        if not cls._instance:
            cls._instance = super().__new__(cls)
            cls._instance.name = name
        return cls._instance


if __name__ == '__main__':
    w = WaitService("at till 9pm")
    print(w.name)

更简洁的Python 3.10+版本 #

使用typing.Self类型替代字符串前向引用，代码可读性更好：

from typing import Optional, Self

class WaitService:
    _instance: Optional[Self] = None
    name: str

    def __new__(cls, name: str = "Default") -> Self:
        if not cls._instance:
            cls._instance = super().__new__(cls)
            cls._instance.name = name
        return cls._instance


if __name__ == '__main__':
    w = WaitService("at till 9pm")
    print(w.name)

内容的提问来源于stack exchange，提问作者Uchenna Nwanyanwu