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

Random832 random832 at fastmail.com
Fri Oct 13 17:02:13 EDT 2017

On Fri, Oct 13, 2017, at 02:30, Nick Coghlan 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.
> It's the second use case where they're problematic, as the fact that
> they're preserved on the class becomes a leaky implementation detail,
> and the lack of a JIT in CPython means they can also end up being
> expensive from a runtime performance perspective.

What about a metaclass that isn't a metaclass? A metaclass can be any
callable and can return any object, e.g. a normal type.

def AutoSlotMeta(name, bases, dct, real_metaclass=type):
    """turn all class variables into slots"""
    dct['__slots__'] = list(dct)
    return real_metaclass(name, bases, dct)

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