如何区分从类内部和类外部使用 __setattr__ 设置的属性？

Question

__setattr__() 中是否有任何方法可以区分来自类内部或子/继承类的属性集，以及来自当前或子类外部的属性集？

我想从“外部”更改设置属性的工作方式，在我制作模块的情况下，我希望用户在设置属性时具有与从类内部设置属性时不同的逻辑。

例如：
i.x = 5 在从类中调用时通常应该赋值为 5，i 是它的一个实例，但是当从另一个类调用时，它应该减去 5 而不是设置为 5 .

Answer 1

有点低级，但你可以使用inspect模块：

import inspect

class A:

    def __init__(self):
        self.__x = 0

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

    @x.setter
    def x(self, value):
        f = inspect.currentframe()
        if 'self' in f.f_back.f_locals and issubclass(type(f.f_back.f_locals['self']), A):
            print('Called from class!')
            self.__x = -value
        else:
            print('Called from outside!')
            self.__x = value

    def fn(self):
        print('Calling A.x from inside:')
        self.x = 10

class B(A):
    def __init__(self):
        super().__init__()

    def fn2(self):
        print('Calling B.x from inside:')
        self.x = 15

a = A()
print("A.x after init:", a.x)
print('Calling A.x from outside')
a.x = 10
print("A.x called from the outside:", a.x)
a.fn()
print("A.x called from the inside:", a.x)

b = B()
print("B.x after init:", b.x)
print('Calling B.x from outside')
b.x = 20
print("B.x called from the outside:", b.x)
b.fn2()
print("B.x called from the inside:", b.x)

打印：

A.x after init: 0
Calling A.x from outside
Called from outside!
A.x called from the outside: 10
Calling A.x from inside:
Called from class!
A.x called from the inside: -10
B.x after init: 0
Calling B.x from outside
Called from outside!
B.x called from the outside: 20
Calling B.x from inside:
Called from class!
B.x called from the inside: -15

Answer 2

使用属性。在类内部，您可以直接分配给基础属性。在外部，x 的分配反而减少了它。

class Foo:
    def __init__(self):
         self._x = 0

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

    @x.setter
    def x(self, value):
        self._x -= value

Answer 3

解决方案可能在于始终在类中使用self.__dict__，而不调用__setattr__ 方法。

例子：

class myClass:

    def __init__(self, value):
        self.__dict__['a'] = value

    def __setattr__(self, name, value):
        print("called from outside")
        if name == 'a':
            self.__dict__[name] = value - 5
        else:
            self.__dict__[name] = value


f = myClass(10)

print(f.a)
# 10

f.a = 20
print(f.a)
# called from outside
# 15