Why to use __setattr__ in python?

Question

I don\'t know for why using __setattr__ instead simple referencing like x.a=1.

I understand this example:

class Rectangle:         


        

Answer 1

class Name(object):
    pass

obj=Name()

#just i have created object two way 
#first

obj.a="first"
print obj.a

#answer= "first"
#second

obj.__setattr__('b','second')
print obj.a
#answer= "second"