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

[Guido van Rossum]

Hi Ronald,

The feature freeze for 3.6 is closing in a few days; 3.6b1 will go out
this weekend. Did you overcome the issue, or does your PEP need to be
postponed until 3.7?

--Guido

On Sun, Jul 24, 2016 at 9:58 PM, Ronald Oussoren <ronaldoussoren at mac.com> wrote:
>
> On 24 Jul 2016, at 13:06, Ronald Oussoren <ronaldoussoren at mac.com> wrote:
>
> …
>
> But on the other hand, that’s why wanted to use PyObjC to validate
> the PEP in the first place.
>
>
> I’ve hit a fairly significant issue with this, PyObjC’s super contains more
> magic than just this magic that would be fixed by PEP 447. I don’t think
> I’ll be able to finish work on PEP 447 this week because of that, and in the
> worst case will have to retire the PEP.
>
> The problem is as follows: to be able to map all of Cocoa’s methods to
> Python PyObjC creates two proxy classes for every Cocoa class: the regular
> class and its metaclass. The latter is used to store class methods. This is
> needed because Objective-C classes can have instance and class methods with
> the same name, as an example:
>
> @interface NSObject
> -(NSString*)description;
> +(NSString*)description
> @end
>
> The first declaration for “description” is an instance method, the second is
> a class method.  The Python metaclass is mostly a hidden detail, users don’t
> explicitly interact with these classes and use the normal Python convention
> for defining class methods.
>
> This works fine, problems starts when you want to subclass in Python and
> override the class method:
>
> class MyClass (NSObject):
>    @classmethod
>    def description(cls):
>       return “hello there from %r” % (super(MyClass, cls).description())
>
> If you’re used to normal Python code there’s nothing wrong here, but getting
> this to work required some magic in objc.super to ensure that its
> __getattribute__ looks in the metaclass in this case and not the regular
> class.  The current PEP447-ised version of PyObjC has a number of test
> failures because builtin.super obviously doesn’t contain this hack (and
> shouldn’t).
>
> I think I can fix this for modern code that uses an argumentless call to
> super by replacing the cell containing the __class__ reference when moving
> the method from the regular class to the instance class. That would
> obviously not work for the code I showed earlier, but that at least won’t
> fail silently and the error message is specific enough that I can include it
> in PyObjC’s documentation.
>
> Ronald
>
>
>
>
>
> Back to wrangling C code,
>
>   Ronald
>
>
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
>
>
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-- 
--Guido van Rossum (python.org/~guido)