[Python-ideas] Restore the __members__ behavior to python3 for C extension writers

[Barry Scott]

> On 19 Jun 2017, at 13:56, Nick Coghlan <ncoghlan at gmail.com> wrote:
> 
> On 19 June 2017 at 04:10, Barry Scott <barry at barrys-emacs.org> wrote:
>> The code does this:
>> 
>>    Py::Object getattro( const Py::String &name_ )
>>    {
>>        std::string name( name_.as_std_string( "utf-8" ) );
>> 
>>        if( name == "value" )
>>        {
>>            return m_value;
>>        }
>>        else
>>        {
>>            return genericGetAttro( name_ );
>>        }
>>    }
>> 
>> Where getattro is called (indirectly) from tp_getattro.
>> 
>> In the python 2 I can tell python that 'value' exists because I provide a value of __members__.
>> 
>> What is the way to tell python about 'value' in the python3 world?
> 
> OK, I think I may understand the confusion now.
> 
> As Thomas noted, the preferred way of informing Python of data
> attributes for types implemented in C is to ask the interpreter to
> automatically create the appropriate descriptor objects by setting the
> `tp_members` slot on the C level *type*, rather than setting
> `__members__` on the instance:
> https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_members
> 
> That approach also works for new-style classes in Python 2:
> https://docs.python.org/2/c-api/typeobj.html#c.PyTypeObject.tp_members <https://docs.python.org/2/c-api/typeobj.html#c.PyTypeObject.tp_members>

I'll see if I can use this to convert existing code that depends on __members__
and report back. 

> 
> I believe this is actually an old-/new-style class difference, so the
> relevant Python 3 change is the fact that the old-style approach
> simply isn't available any more.
> 
> So if you use tp_members and tp_getset to request the creation of
> suitable descriptors, then the interpreter will automatically take
> care of populating the results of `dir()` correctly. However, if
> you're genuinely dynamically adding attributes in `__getattr__`, then
> you're going to need to add code to report them from `__dir__` as
> well.
> 
> Cheers,
> Nick.
> 
> -- 
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
> 

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