[Python-Dev] What is the design purpose of metaclasses vs code generating decorators? (was Re: PEP 557: Data Classes) (original) (raw)

Ethan Furman ethan at stoneleaf.us
Sat Oct 14 11:49:30 EDT 2017

On 10/14/2017 07:37 AM, Martin Teichmann wrote:

Things that will not work if Enum does not have a metaclass:

list(EnumClass) -> list of enum members dir(EnumClass) -> custom list of "interesting" items len(EnumClass) -> number of members member in EnumClass -> True or False - protection from adding, deleting, and changing members - guards against reusing the same name twice - possible to have properties and members with the same name (i.e. "value" and "name") In current Python this is true. But if we would go down the route of PEP 560 (which I just found, I wasn't involved in its discussion), then we could just add all the needed functionality to classes. I would do it slightly different than proposed in PEP 560: classmethods are very similar to methods on a metaclass. They are just not called by the special method machinery. I propose that the following is possible: >>> class Spam: ... @classmethod ... def getitem(self, item): ... return "Ham" >>> Spam[3] Ham this should solve most of your usecases.

The problem with your solution is you couldn't then have a getitem for the instances -- it's an either/or situation. The problem with PEP 560 is that it doesn't allow the class definition protections that a metaclass does.

-- Ethan

More information about the Python-Dev mailing list