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

The first use of __setattr__ is to be overwritten in a class definition.

class Foo ( object ) :

    def __init__ ( self ) :
        self.x = 'looser'

    def __setattr__ ( self, name, value ) :
        if name == 'x' :
            print( 'Hello, {} !'.format( value ) )
            self.x = value

Problem, Foo() will print an infinite sequence of :

'Hello, looser !'

There comes the second use which is that, when you're doing that, you can call setattr from the parent class (object by default) to avoid infite recursion :

class Foo ( object ) :

    def __setattr__ ( self, name, value ) :
        self.bar( name, value )
        object.__setattr__( self, name, value )

    def bar ( self, name, value ) :
        print( 'Setting {} to {}.'.format( name, value ) )

And therefore :

f = Foo()

>> 'Hello, winner !'

f.x

>> 'winner'