What does __setattr__ do in this python code?

Question

this is my code:

class fun:

    def __getattr__(self,key):
        return self[key]

    def __setattr__(self,key,value):
        self[key] = value+1
a = fun()
a['x']=1
print a['x']

and the error is :

AttributeError: fun instance has no attribute '__getitem__'

when i change it to :

class fun:

    def __getattr__(self,key):
        return self.key

    def __setattr__(self,key,value):
        self.key = value+1
a = fun()
a.x=1
print a.x

the error is :

RuntimeError: maximum recursion depth exceeded

what can I do, I want get 2

Answer 1

The problem is that self.key = ... invokes __setattr__ , so you end up in an infinite recursion. To use __setattr__ , you have to access the object's field some other way. There are two common solutions:

def __setattr__(self,key,value):
    # Access the object's fields through the special __dict__ field
    self.__dict__[key] = value+1

# or...

def __init__(self):
    # Assign a dict field to access fields set via __[gs]etattr__
    self.attrs = {}

def __setattr__(self,key,value):
    self.attrs[key] = value+1

Answer 2

It's a typo.

You want to implement the special method __setattr__ , and not __serattr__ which has no special meaning.

Answer 3

Firstly, the method is called __setattr__() . It is when an attribute assignment is attempted. Such as when you do:

self[key] = value+1

...making your particular call (infinitely) recursive!

A better way to do this would be to derive your class from object , a so-called new-style class and call the base class:

class fun(object):

    def __setattr__(self,key,value):
        super(fun, self).__setattr__(key, value + 1)

a = fun()
a.x=1
print a.x

I removed your __getattr__() implementation, since it did nothing of any value.