object().__dict_

[Duncan Booth]

Pavel Volkov <sailor at lists.xtsubasa.org> wrote:

> There are some basics about Python objects I don't understand.
> Consider this snippet:
> 
>>>> class X: pass
> ... 
>>>> x = X()
>>>> dir(x)
> ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__',
> '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__',
> '__hash__', '__init__', '__le__', '__lt__', '__module__', '__ne__',
> '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__',
> '__sizeof__', '__str__', '__subclasshook__', '__weakref__']
>>>> x.foo = 11
> 
> And now I want to make a simple object in a shorter way, without
> declaring X class:
> 
>>>> y = object()
>>>> dir(y)
> ['__class__', '__delattr__', '__dir__', '__doc__', '__eq__',
> '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__',
> '__init__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__',
> '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
> '__subclasshook__'] 
>>>> y.foo = 12
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: 'object' object has no attribute 'foo'
> 
> The attribute list is different now and there's no __dict__ and the
> object does not accept new attributes.
> Please explain what's going on.
> 
> 
Not all classes have a __dict__ attribute. Mostly builtin classes (e.g. 
tuple, list, int, ...), but also if you have an class using __slots__ 
which subclasses a class with no __dict__ it won't have a __dict__.

Subclasses don't remove attributes, they only add them (although in 
Python you can bend that rule it still applies here). Therefore for any 
classes to not have a __dict__ attribute the ultimate base class 
'object' has to not have a __dict__.

The consequence, as you found out, is that you cannot add attributes to 
an instance of 'object()', you have to create at least an empty subclass 
which doesn't include a `__slots__` attribute to get a class that can 
accept arbitrary attributes.


-- 
Duncan Booth