[Python-ideas] PEP 447: Adding type.__getdescriptor__

[Nick Coghlan]

On 1 December 2017 at 19:15, Ronald Oussoren <ronaldoussoren at mac.com> wrote:

> Maybe, but how would this work with super()? Super walks the MRO of type of
> the instance, but skips the class on the MRO.  This is not equivalent to
> walking the MRO of the second class on the MRO when you use multiple
> inheritance,
>
> This also has some other disadvantages. The first is that
> tp.__getdescriptor__ would replace the default behaviour for the entire MRO
> and it would be possible to have different behavior for classes on the MRO.
> The second, minor. one is that __getdescriptor__ would have to reimplement
> the default logic of walking the MRO, but that logic is fairly trivial.

I believe those can both be addressed by structuring the override a
little differently, and putting it at the level of individual
attribute retrieval on a specific class (rather than on all of its
subclasses):

    def _PyType_Lookup(tp, name):
        mro = tp.mro()
        assert isinstance(mro, tuple)

        for base in mro:
           assert isinstance(base, type)

           try:
               getdesc = base.__dict__["__getdescriptor__"]
           except KeyError:
               try:
                   return base.__dict__[name]
               except KeyError:
                   pass
           else:
               try:
                   return getdesc(tp, base, name)
               except AttributeError:
                   pass

        return None

In that version, the __getdescriptor__ signature would be:

    def __getdescriptor__(dynamic_cls, base_cls, attr):
        ...

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia