[Python-Dev] __objclass__ documentation

[Nick Coghlan]

On 13 Oct 2013 08:27, "Ethan Furman" <ethan at stoneleaf.us> wrote:
>
> It was pointed in Issue16938[1] that __objclass__ is not documented anywhere.
>
> Is the following an appropriate description? (in Doc/reference/datamodel.rst in user-defined functions)
>
>
>
>       +-------------------------+-------------------------------+-----------+
>       | :attr:`__objclass__`    | The class this object belongs |           |
>       |                         | to; useful when the object is |           |
>       |                         | a descriptor, or a virtual or |           |
>       |                         | dynamic class attribute, and  |           |
>       |                         | it's __class__ attribute does |           |
>       |                         | not match the class it is     |           |
>       |                         | associated with, or it is not |           |
>       |                         | in that class' ``__dict__``.  |           |
>       +-------------------------+-------------------------------+-----------+

I think this is inaccurate. The meaning of __objclass__ as described
in PEP 252 is to denote unbound callable descriptors that only work
with a specific type. That's why it's mostly gone in Python 3.

It should be documented, but the documentation should say something like:

__objclass__: Indicates this callable requires an instance of the
given type (or a subclass) as its first positional argument. For
example, CPython sets this for unbound methods that are implemented in
C rather than Python.

The offending block of code in the inspect module
(http://hg.python.org/cpython/file/default/Lib/inspect.py#l366) that
prompted http://bugs.python.org/issue16938 just looks flat out broken
to me. There's *zero* reason to expect that __class__ on the result of
retrieving an attribute from an object will appear in the MRO for that
object. I just missed that detail when reviewing
http://bugs.python.org/issue19030 :)

I'll reply on the tracker with some additional details/suggestions.

Cheers,
Nick.