Python: avoid setattr loop using dict, shouldn't it loop anyway?

Question

I know that you should be aware while redefine setattr method that you could end in a loop.

I know there's two solution to avoid loops:

1) using object superclass, calling :

object.__setattr__(instance,attr,value)

2) using the class dict:

self.__dict__[attr] = value

My question is, shouldn't the __dict__ solution end up in a loop too? and why it isn't?

Is because that way we're calling the __setattr__ of the __dict__ object (everything is an object) or what?

And if so, shouldn't it work the same for everything?

a[b] = c doesn't set an attribute. It sets an item. It's a different thing both conceptually and w.r.t. dunder methods involved. — user395760
– user395760, Commented May 30, 2013 at 16:01

mgilson · Accepted Answer · 2013-05-30 16:02:06Z

5

Why would you expect it to end up in a loop?

instance.__dict__[attr] = value is basically what object.__setattr__(instance,attr,value) does (for normal attributes). Note that __dict__[attr] = whatever does not call __setattr__ at all. It calls __setitem__ which is a different method entirely.

answered May 30, 2013 at 16:02

mgilson

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3 Comments

user395760 Over a year ago

It also calls it on an entirely different object.

mgilson Over a year ago

@delnan -- True. Of course, using __getattribute__ you can get into the infinite loop using self.__dict__, but ...

Mattia Over a year ago

So it is because is calling setitem or because is a different object? thanks guys

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