[Python-Dev] PEP 560 vs metaclass' class definition protections [was Re: What is the design purpose of metaclasses ...]

[Nick Coghlan]

On 15 October 2017 at 02:14, Ethan Furman <ethan at stoneleaf.us> wrote:

> On 10/14/2017 08:57 AM, Ivan Levkivskyi wrote:
>
>> On 14 October 2017 at 17:49, Ethan Furman wrote:
>>
>
> The problem with PEP 560 is that it doesn't allow the class definition
>>>
>> >> protections that a metaclass does.
>
>>
>> Since the discussion turned to PEP 560, I can say that I don't want this
>>
> > to be a general mechanism, the PEP rationale section gives several
> specific
> > examples why we don't want metaclasses to implement generic class
> > machinery/internals.
>
>>
>> Could you please elaborate more what is wrong with PEP 560 and what do you
>>
> > mean by "class definition protections"
>
> Nothing is wrong with PEP 560.  What I am referring to is:
>
> class MyEnum(Enum):
>    red = 0
>    red = 1
>
> The Enum metaclass machinery will raise an error at the "red = 1" line
> because it detects the redefinition of "red". This check can only happen
> during class definition, so only the metaclass can do it.
>

That's not necessarily an inherent restriction though - if we did decide to
go even further in the direction of "How do we let base classes override
semantics that currently require a custom metaclass?", then there's a
fairly clear parallel between "mcl.__init__/bases.__init_subclass__" and
"mcl.__prepare__/bases.__prepare_subclass__".

OTOH, if you have multiple bases with competing __prepare__  methods you
really *should* get a metaclass conflict, since the class body can only be
executed in one namespace.

Cheers,
Nick.

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