python - 动态添加/覆盖property属性的setter和getter

Question

我需要使用/模仿语法糖语法在子类中动态装饰 getter 和 setter 对方法。

我正在努力解决 setter 的实现问题。

class A:

    def __init__(self, x):
        print('init')
        self.__x = x

    @property
    def x(self):
        print('getter')
        return self.__x

    @x.setter
    def x(self, v):
        print('setter')
        self.__x = v


class Dec:
    def __init__(self):
        print('init - dec')

    def __call__(self, cls):
        c = type('A_Dec', (cls,), {})
        # super-init
        setattr(c, '__init__', lambda sub_self, x: super(type(sub_self), sub_self).__init__(x))
        # getter
        setattr(c, 'x', property(lambda sub_self: super(type(sub_self), sub_self).x))
        
        # setter - see below

        return c

dec_A = Dec()(A)
dec_a = dec_A('p')
print(dec_a.x)

输出

init - dec
init
getter
p

如果我尝试使用以下方法在 Dec、dec_a.x = 'p' 中实现 setter 方法，我会收到以下错误:

    # setter-statements of __call__

    # Attempt 1
    setattr(c, 'x', property(fset=lambda sub_self, v: super(type(sub_self), sub_self).x(v)))
    # AttributeError: unreadable attribute
    
    # Attempt 2 - auxiliary function
    def m(sub_self, v):
       print('--> ', sf, super(type(sub_self), sub_self))
       super(type(sub_self), sub_self).x = v
    
    # Attempt 2.A
    setattr(c, 'x', eval('x.setter(m)'))
    # NameError: name 'x' is not defined
    
    # Attempt 2.B
    setattr(c, 'x', property(fset=lambda sf, v: m(sf, v)))
    # AttributeError: unreadable attribute
    
    # Attempt 2.C: !! both at once, `fget`and `fset` so, in case, comment the getter in the above code to avoid conflicts
    setattr(c, 'x', property(fget=lambda sub_self: super(type(sub_self), sub_self).x, fset=m))
    # AttributeError: 'super' object has no attribute 'x'
    
    # Attempt 2.D
    p = property(fget=lambda sub_self: super(type(sub_self), sub_self).x, fset=m)
    setattr(c, 'x', p)
    # AttributeError: 'super' object has no attribute 'x'

尝试 1 会引发错误，因为(我猜)用括号设置属性。因此，在尝试 2 中，我使用了辅助函数，因为 lambda 不允许初始化、“=”语句，但同样没有成功。

• 有没有办法动态模仿属性 getter/setter 装饰器？ (可能没有额外的导入) 还有其他方法吗？

• 额外:为什么 super 在没有属性的情况下不起作用？ super().x(v) -> TypeError: super(type, obj): obj 必须是 type 的实例或子类型

编辑:

• 额外答案:来自文档:零参数形式仅适用于类定义内部[...]
• 使用 python3.9

Answer 1

属性 setter 未正确设置。为了形象化这一点，如果没有为属性显式设置 setter，则该属性将变为只读，如 documented 。

class Parrot:
    def __init__(self):
        self._voltage = 100000

    @property
    def voltage(self):
        """Get the current voltage."""
        return self._voltage

The @property decorator turns the voltage() method into a “getter” for a read-only attribute with the same name

假设我们有这个:

class A:
    def __init__(self, x):
        self.__x = x

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

a = A(123)

print(a.x)  # will display "123"
a.x = 456  # will display "AttributeError: can't set attribute"

在原始代码中，您创建了一个新类型A_Dec。您显式设置 getter:

# getter
setattr(c, 'x', property(lambda sub_self: super(type(sub_self), sub_self).x))

但是您没有显式设置任何 setter，从而使 x 属性变为只读。这会导致此代码出错:

dec_a.x = 'new value!'  # will display "AttributeError: can't set attribute"

解决方案1

不要明确定义 getter。这样，对 x 的所有访问都将委托(delegate)给实际的类 A。

解决方案2

如果定义了 getter，那么还要定义 setter。

...
class Dec:
    ...
    def __call__(self, cls):
        ...
        # setter
        x_property = getattr(c, 'x')
        x_setter = getattr(x_property, 'setter')
        setattr(c, 'x', x_setter(lambda sub_self, v: super(type(sub_self), type(sub_self)).x.fset(sub_self, v)))
        ...
...

• c.x.setter 的用法如 documented :

  A property object has getter, setter, and deleter methods usable as decorators

• .fset 的用法如 documented :

  fset is a function for setting an attribute value... The returned property object also has the attributes fget, fset, and fdel corresponding to the constructor arguments.

因此添加以下行将会成功:

dec_a.x = 'new value!'
print(dec_a.x)

输出:

setter
getter
new value!

更多引用:

• https://newbedev.com/how-to-call-a-property-of-the-base-class-if-this-property-is-being-overwritten-in-the-derived-class
• https://gist.github.com/Susensio/979259559e2bebcd0273f1a95d7c1e79