difference between __setattr__ and __dict__

Question

if I want to set an attribute for a instance：

I can use __setattr__(key, value) or self.__dict__[key]=value

I want to know how __setattr__ works, if __setattr__ just set the k,v in __dict__, whats the differencet between two ways? What is the meaning of setattr's existence？

Answer 1

while __dict__ is an internal dictionary that holds all of their internal variables, __setattr__ is actually a method that is always called when you try to set an attribute, and therefore, you can intercept these actions and act according.

A sample code can help you to understand the difference:

class Foo:
    def __setattr__(self, key, value):
        print(f'Setting {key} to {value}')
        self.__dict__[key] = value

f = Foo()
f.__dict__['test'] = 1
f.__setattr__('test', 2)
f.test = 3

The output would be:

Setting test to 2
Setting test to 3

And the reason is that by accessing __dict__, you are directly accessing the internal object dictionary and the __setattr__ method is skipped.

Answer 2

__setattr__(key, value) is the function which gets called by setattr(instance, key, value).

__setattr__(key, value) typically will act the same as self.__dict__[key]= value unless the class the __setattr__ method in the function has been overridden in some way.

This might help docs

class MyClass(object):
    def __setattr__(self, key, value):
        print(f"setting {key}={value}")

if __name__ == "__main__":
    m = MyClass()
    m.a = 3
    m.b = 6
    m.__dict__.update({"c": 7})
    print(f"{m.c=}")
    print(f"{m.__dict__=}")

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