动态添加/覆盖属性属性的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。

我正在为 setter 实现而苦苦挣扎。

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

如果我尝试在Dec、dec_a.x = 'p' 中实现 setter 方法，使用以下方法会收集到以下错误：

    # 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'

Attempt 1 引发错误，因为（我猜）用括号设置属性。所以在尝试2中我使用了一个辅助函数，因为lambda不允许初始化，'='语句，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 没有属性就不能工作？ super().x(v) -> TypeError: super(type, obj): obj must be an instance or subtype of type

编辑：

• Extra 的答案：来自文档：零参数形式仅适用于类定义[...]
• 使用 python3.9

Answer 1

参考文献

• doc super
• doc property
• doc descriptor

重要

• 与python 3.9 合作（否则可能与super 或其他人合作）
• 从文档中调用 零参数形式 [super()] 仅适用于类定义 [...]。

简要案例2.3代表问题的解决方案，其他案例研究显示可能的替代方案和（也许）有关如何达到它的有用信息

---

步骤 0：父类（配备描述符协议的引用类）

class A:

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

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

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

第 1 步：审核 - 静态覆盖 getter/setter，两种方式

方式 1.1 - 语法糖

class B(A):

    def __init__(self, x): super().__init__(x)

    @property
    def x(self): return super().x # still not sure if there isn't a better way

    @x.setter
    def x(self, v): super(B, B).x.fset(self, v)

b = B('x')
print(b.x)
b.x = 'xx'
print(b.x)

# Output
x
xx

方式 1.2 - 属性作为类属性

class C(A):

    def __init__(self, x): super().__init__(x)

    def x_read(self): return super().x

    def x_write(self, v): super(C, C).x.fset(self, v) # notice the arguments of super!!!

    x = property(x_read, x_write)

c = C('x')
print(c.x)
c.x = 'xx'
print(c.x)

# Output
x
xx

第 2 步：动态覆盖 getter/setter - 两种方式

方式 2.1 静态装饰器（这是解决方案在静态设置中的样子）

class DecStatic:

    def __init__(self):
        print('init - static dec')

    def __call__(self, cls):
        class A_Dec(cls):
            def __init__(sub_self, x):
                super().__init__(x)

            @property
            def x(sub_self):
                return super().x

            @x.setter
            def x(sub_self, v):
                super(type(sub_self), type(sub_self)).x.fset(sub_self, v)

        return A_Dec

dec_A = DecStatic()(A)
dec_a = dec_A('x')
print(dec_a.x)
dec_a.x = 'xx'
print(dec_a.x)

# Output
init - static dec
x
xx

方式 2.2 动态装饰器 - 将属性覆盖为类属性

class DecDynamicClsAttr:
    def __init__(self):
        print('init - dynamic dec - class attr')

    def __call__(self, cls):
        DecA = type('DecA', (cls,), {})
        # super-init
        setattr(DecA, '__init__', lambda sub_self, x: super(type(sub_self), sub_self).__init__(x))
        # property
        setattr(DecA, 'x', property(fget=lambda sub_self: super(type(sub_self), sub_self).x, fset=lambda sub_self, v: super(type(sub_self), type(sub_self)).x.fset(sub_self, v)))

        return DecA

dec_A = DecDynamicClsAttr()(A)
dec_a = dec_A('x')
print(dec_a.x)
dec_a.x = 'xx'
print(dec_a.x)

# Output
init - dynamic dec - class attr
x
xx

方式 2.3 动态装饰器 - （苦涩的）语法糖语法 (

class DecDynamicSS:
    def __init__(self):
        print('init - dynamic dec - syntactic sugar')

    def __call__(self, cls):
        DecA = type('A_Dec', (cls,), {})
        # super-init
        setattr(DecA, '__init__', lambda sub_self, x: super(type(sub_self), sub_self).__init__(x))

        # getter
        setattr(DecA, 'x', property(lambda sub_self: super(type(sub_self), type(sub_self)).x.fget(sub_self)))
        # setter
        setattr(DecA, 'x', DecA.x.setter(lambda sub_self, v: super(type(sub_self), type(sub_self)).x.fset(sub_self, v)))

        return DecA

dec_A = DecDynamicSS()(A)
dec_a = dec_A('x')
print(dec_a.x)
dec_a.x = 'xx'
print(dec_a.x)

# Output
init - dynamic dec - syntactic sugar
x
xx

备注

• 从 2.2 开始，很明显，以 getter/setter 描述符为特征的属性绑定到类而不是实例，所以在 2.3 中，我们也应该跟踪类：

     `x.setter` --> `cls.x.setter`
  

• 感谢 @chepner 的评论和@Niel Godfrey Ponciano 的回答，感谢他们提供的有用信息

Answer 2

属性设置器未正确设置。为了可视化这一点，如果没有为属性显式设置 setter，则该属性将变为只读，为 documented。

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

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

@property 装饰器将 voltage() 方法转换为具有相同名称的 只读属性 的“getter”

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"

在您的原始代码中，您创建了一个新类型A_Dec。您明确设置了 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 属性为只读。这会导致这段代码出错：

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，所有对x 的访问都将委托给实际类A。

解决方案 2

如果你定义了getter，那么也要定义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：

  属性对象具有可用作装饰器的getter、setter和deleter方法

• .fset的用法如documented：

  fset是设置属性值的函数...返回的属性对象还有fget、fset、@987654343属性@对应构造函数参数。

所以添加下面几行就成功了：

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

输出：

setter
getter
new value!

更多参考资料：

• 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