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