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

Kerem · Accepted Answer · 2016-02-06 19:06:44Z

7

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

edited Feb 6, 2016 at 19:06

Kerem

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answered Mar 19, 2011 at 9:47

Marcelo Cantos

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Comments

Mark Byers · Accepted Answer · 2011-03-19 09:37:20Z

3

It's a typo.

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

answered Mar 19, 2011 at 9:37

Mark Byers

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Comments

johnsyweb · Accepted Answer · 2011-03-19 09:48:39Z

1

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.

answered Mar 19, 2011 at 9:48

johnsyweb

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