[Python-Dev] PEP 447: Add __getdescriptor__ to metaclasses

Nick Coghlan ncoghlan at gmail.com
Sun Jul 24 06:37:17 EDT 2016

On 23 July 2016 at 22:26, Ronald Oussoren <ronaldoussoren at mac.com> wrote:
> I’m currently working on getting the patch in 18181 up-to-date w.r.t. the
> current trunk, the patch in the issue no longer applies cleanly. After that
> I’ll try to think up some tests that seriously try to break the new
> behaviour, and I want to update a patch I have for PyObjC to make use of the
> new functionality to make sure that the PEP actually fixes the issues I had
> w.r.t. builtin.super’s behavior.

You may also want to check compatibility with Martin's patch for PEP
487 (__init_subclass__ and __set_name__) at

I don't *think* it will conflict, but "try it and see what happens" is
generally a better idea for the descriptor machinery than assuming
changes are going to be non-conflicting :)

> What is the best way forward after that? As before this is a change in
> behavior that, unsurprisingly, few core devs appear to be comfortable with
> evaluating, combined with new functionality that will likely see little use
> beyond PyObjC.

You may want to explicitly ping the
https://github.com/ipython/traitlets developers to see if this change
would let them do anything they currently find impractical or

As far as Mark's concern about a non-terminating method definition
goes, I do think you need to double check how the semantics of
object.__getattribute__ are formally defined.

    >>> class Meta(type):
    ...     def __getattribute__(self, attr):
    ...         print("Via metaclass!")
    ...         return super().__getattribute__(attr)
    >>> class Example(metaclass=Meta): pass
    >>> Example.mro()
    Via metaclass!
    [<class '__main__.Example'>, <class 'object'>]

Where the current PEP risks falling into unbounded recursion is that
it appears to propose that the default type.__getdescriptor__
implementation be defined in terms of accessing cls.__dict__, but a
normal Python level access to "cls.__dict__" would go through the
descriptor machinery, triggering an infinite regress.

The PEP needs to be explicit that where "cls.__dict__" is written in
the definitions of both the old and new lookup semantics, it is *not*
referring to a normal class attribute lookup, but rather to the
interpreter's privileged access to the class namespace (e.g. direct
'tp_dict' access in CPython).


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

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