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

Ethan Furman ethan at stoneleaf.us
Sat Oct 14 12:14:34 EDT 2017

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.


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