类成员和方法

class A:
    x = None  # type: float
    def __init__(self, x: float) -> None:
        """
        self should not be annotated
        init should be annotated to return None
        """
        self.x = x
    
    @classmethod
    def from_int(cls, x: int) -> 'A': 
        """
        cls should not be annotated
        Use forward reference to refer to current class with string literal 'A'
        """
        return cls(float(x))

由于在定义函数时评估注释，因此需要当前类的前向引用。当引用可导致循环导入的类时，也可以使用前向引用。