> On 19 Jun 2017, at 13:56, Nick Coghlan <ncogh...@gmail.com> wrote:
>
> On 19 June 2017 at 04:10, Barry Scott <ba...@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
> <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>
This is not useful as the values in my use cases are typically taken from a C
or C++ objects that holds the master copy. The python class is a facade that is
forwarding to a embedded object typically. And the values return might switch
type.
Being a PyString or None for example.
>
> 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.
tp_getset might work. I'll have to a largest block of time experiment with it
and think about C++ API for it.
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia
>
_______________________________________________
Python-ideas mailing list
Python-ideas@python.org
https://mail.python.org/mailman/listinfo/python-ideas
Code of Conduct: http://python.org/psf/codeofconduct/
