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(Written in Python shell)

>>> o = object() >>> o.test = 1 Traceback (most recent call last): File "<pyshell#45>", line 1, in <module> o.test = 1 AttributeError: 'object' object has no attribute 'test' >>> class test1: pass >>> t = test1() >>> t.test Traceback (most recent call last): File "<pyshell#50>", line 1, in <module> t.test AttributeError: test1 instance has no attribute 'test' >>> t.test = 1 >>> t.test 1 >>> class test2(object): pass >>> t = test2() >>> t.test = 1 >>> t.test 1 >>>

Why doesn't object allow you to add attributes to it?

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Notice that an object instance has no __dict__ attribute:

>>> dir(object()) ['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__']

An example to illustrate this behavior in a derived class:

>>> class Foo(object): ... __slots__ = {} ... >>> f = Foo() >>> f.bar = 42 Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'Foo' object has no attribute 'bar'

Quoting from the docs on slots:

[...] The __slots__ declaration takes a sequence of instance variables and reserves just enough space in each instance to hold a value for each variable. Space is saved because __dict__ is not created for each instance.

EDIT: To answer ThomasH from the comments, OP's test class is an "old-style" class. Try:

>>> class test: pass ... >>> getattr(test(), '__dict__') {} >>> getattr(object(), '__dict__') Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'object' object has no attribute '__dict__'

and you'll notice there is a __dict__ instance. The object class may not have a __slots__ defined, but the result is the same: lack of a __dict__, which is what prevents dynamic assignment of an attribute. I've reorganized my answer to make this clearer (move the second paragraph to the top).

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So, you are saying that the object class is using __slots__ in one way or another, to prevent assignment of instance members. Is this an assumption or a fact (i.e. is it in the implementation of object)? Because the mere absence of _\dict__ from an object instance is not indicative. Instances of the OP's test1 class show the same missing __dict__ when called with dir(), but allow nevertheless assignment of instance members.
@ThomasH, please see the note I added to the answer.
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Good question, my guess is that it has to do with the fact that object is a built-in/extension type.

>>> class test(object): ... pass ... >>> test.test = 1 >>> object.test = 1 Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: can't set attributes of built-in/extension type 'object'

IIRC, this has to do with the presence of a __dict__ attribute or, more correctly, setattr() blowing up when the object doesn't have a __dict__ attribute.

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