Why can't you add attributes to object in python? [duplicate]

Question

(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?

Answer 1

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).

Answer 2

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.