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？
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.
__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