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pythonmagic-methodssetattr

How to differentiate between a property being set with ___setattr__ from inside the class and outside the class?

Is there any way in __setattr__() to differentiate between an attribute set from inside the class or a child/inheriting class, and an attribute set from outside the current or a child class?

I want to change how setting attributes works from the "outside", in my case of making a module, I want the user to have different logic when setting a attribute than when it's set from inside the class.

For example:
i.x = 5 should assign 5 normally when called from within the class and i is a instance of it, but when called from another class it should, say, subtract 5 instead of set to 5.

Solution

  • A bit lowlevel, but you could use inspect module:

    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)

    Prints:

    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