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

Martin Teichmann lkb.teichmann at gmail.com
Fri Oct 13 05:35:17 EDT 2017

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. Especially for ABCs it would be much better, because the fact that a class is an ABC is explicitly not inherited - its entire reason of existence is to be inherited with all the abstractness going away. And yet, currently the concrete class will still inherit from ABC. The following way of writing ABCs looks much nicer to me:

@abstractclass
class Spam:
    @abstractmethod
    def ham(self):
         ...

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

I'm not an expert on ORMs, but they seem to me a lot like data classes in the context of this discussion.

I am aware that currently some things are easier done with metaclasses. But given that decorators can create an entirely new class if they like, they have all the power to do what they want, and even in a way much easier understood by people.

As an example, here the autoslot decorator:

def autoslot(cls):
    """turn all class variables into slots"""
    cls.__slots__ = list(cls.__dict__)
    return type(cls.__name__, cls.__bases__, class.__dict__)

So I am personally more and more leaning towards a metaclass-free future.

Cheers

Martin

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