setattr() on "object" instance

R. David Murray rdmurray at
Mon Mar 16 15:41:47 CET 2009

Sean DiZazzo <half.italian at> wrote:
> Why is it that you can setattr() on an instance of a class that
> inherits from "object", but you can't on an instance of "object"
> itself?
> >>> o = object()
> >>> setattr(o, "x", 1000)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: 'object' object has no attribute 'x'
> >>> class Object(object):pass
> ...
> >>> o = Object()
> >>> setattr(o, "x", 1000)
> >>> o.x
> 1000
> I notice that the first example's instance doesn't have a __dict__.
> Is the second way the idiom?

The lack of a __dict__ is why you can't set the attribute.
I've occasionally wanted to use instances of object as holders of
arbitrary attributes and wondered why I couldn't (from a language design
perspective).  But that was only for testing.  In real code I think I'd
always want a fully defined class.

R. David Murray 

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