禁止在Python中将类成员设置为“无”

Question

Python中有没有一种方法禁止将类成员设置为__init__之外的None

class Dummy:
    def __init__(self, x):
        if x is not None:
            self.x = x
        else:
            raise Exception("x cannot be None")

d = Dummy("foo")
d.x = None

在我的代码中，我有类型提示，但这些提示并没有强制执行，所以说x只能是str并不会真正改变允许的内容

Answer 1

您需要一个^{}：

class Dummy:
    def __init__(self, x):
        self.x = x

    @property
    def x(self):
        return self._x

    @x.setter
    def x(self, value):
        if value is None:
            raise Exception("x cannot be None")
        self._x = value


d = Dummy(8)
d.x = 16
d.x = None  # Raises

Answer 2

注意：我将此问题解释为防止所有属性被设置为__init__之外的None，尽管我在下面添加了一个选项来保护某些指定属性

这个怎么样？将__setattr__方法作为object.__setattr__的包装器

class Dummy:
    def __init__(self, x):
        self._None_forbidden = False
        self.z = None  # this will work
        self._None_forbidden = True

    def __setattr__(self, k, v):
        if v is None and self._None_forbidden:
            raise ValueError("cannot set attribute to None")
        object.__setattr__(self, k, v)


d = Dummy("foo")
print(d.z)  # None
d.y = 2
print(d.y)  # 2
d.x = None  # raises ValueError

如果您只是想保护某些属性，那么可以将self._None_forbidden设置为一组不允许设置为“无”的属性名称。例如：

class Dummy:
    def __init__(self, x):
        self._None_forbidden = set()
        self.z = None  # this will work
        self._None_forbidden.add("z")

    def __setattr__(self, k, v):
        if v is None and k in self._None_forbidden:
            raise ValueError("cannot set attribute to None")
        object.__setattr__(self, k, v)


d = Dummy("foo")
print(d.z)  # None
d.y = 2
print(d.y)  # 2
d.x = None
print(d.x)  # None
d.z = 4
print(d.z)  # 4
d.z = None  # raises ValueError

显然，如果调用方操纵了_None_forbidden，那么这是可以避免的，但是他们应该知道他们正在做一些不受支持的事情