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

Ethan Furman ethan at stoneleaf.us
Fri Oct 13 15:10:50 EDT 2017

On 10/13/2017 02:35 AM, Martin Teichmann wrote:
>> Metaclasses currently tend to serve two distinct purposes:
>> 1. Actually altering the runtime behaviour of a class and its children in
>> non-standard ways (e.g. enums, ABCs, ORMs)
>> 2. Boilerplate reduction in class definitions, reducing the amount of code
>> you need to write as the author of that class
>> Nobody has a problem with using metaclasses for the first purpose - that's
>> what they're for.
> I am that nobody. The examples you give would be much nicer solved
> with decorators.

> The same holds for enums. Inheriting from enums is possible, but
> weird, given that you cannot add new enums to it. So, especially when
> comparing to the dataclasses, the following looks appealing to me:
>      @enum
>      class Breakfast:
>          spam = 0
>          ham = 1

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")


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