[Python-Dev] objclass documentation (original) (raw)

Ethan Furman ethan at stoneleaf.us
Sun Oct 13 16:15:28 CEST 2013

On 10/13/2013 04:44 AM, Nick Coghlan wrote:

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.

PEP 252 has this comment:

 [Editor's note: the ideas described in this PEP have been incorporated
  into Python.  The PEP no longer accurately describes the implementation.]

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 :)

Actually, it's the other way around -- the code currently in inspect.py at that point was prompted by this issue (and tracked in Issue 19030). Interestingly enough, with the current changes in 16938 I can remove the objclass attribute I had added to Enum to get around this.

-- Ethan

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