>>> class Foo:
...    __slots__ = ['foo']
...    foo = None
... 
>>> x = Foo()
>>> x.foo
>>> x.foo = 5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Foo' object attribute 'foo' is read-only

This sort of thing is true of any slotted class with class attributes:

>>> class X:
...     __slots__ = ()
...     foo = None
... 
>>> X().foo = "hello"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'X' object attribute 'foo' is read-only

I think what needs to happen is the error message needs to be improved in both cases.

> This sort of thing is true of any slotted class with class attributes:
> 
> >>> class X:
> ...     __slots__ = ()
> ...     foo = None
> ... 
> >>> X().foo = "hello"
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: 'X' object attribute 'foo' is read-only

But that's quite different. In your example, the attribute can't be set
on the instance, while it should be in mine (since 'foo' is amongst the
__slots__).

So would you prefer to see

>>> x.foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: foo
>>> x.foo = 5
>>> x.foo
5
?

> So would you prefer to see
> 
> >>> x.foo
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: foo
> >>> x.foo = 5
> >>> x.foo
> 5
> ?

No, I would prefer to see

True
>>> x.foo = 5
>>> x.foo
5

... exactly as in the same class without a __slots__

(the whole point of class attributes being to set default values without
complicating the constructor)

I'm afraid that's not (easily) possible. Using __slots__ implicitly gives classes attributes, so what you are seeing is the effect of toying with that. It's not really possible to pretend there are different attributes on the class than the descriptors. We can:

- Silently override slot descrs with attributes (as currently done).
- Warn or error out on conflicts.
- Just override other class attributes with the descrs.

> I'm afraid that's not (easily) possible. Using __slots__ implicitly
> gives classes attributes, so what you are seeing is the effect of
> toying with that. It's not really possible to pretend there are
> different attributes on the class than the descriptors. We can:
> 
> - Silently override slot descrs with attributes (as currently done).
> - Warn or error out on conflicts.
> - Just override other class attributes with the descrs.

Erroring on conflicts would seem reasonable. If that threatens to break
existing code, at least improving the current error message would be
nice.

New changeset 45b63a8a76c9 by Benjamin Peterson in branch 'default':
complain when a class variable shadows a name in __slots__ (closes #12766)
http://hg.python.org/cpython/rev/45b63a8a76c9