[Python-ideas] A way out of Meta-hell (was: A (meta)class algebra) (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sun Feb 15 11:08:18 CET 2015

On 14 February 2015 at 20:23, Petr Viktorin <encukou at gmail.com> wrote:

On Sat, Feb 14, 2015 at 10:34 AM, Martin Teichmann <lkb.teichmann at gmail.com> wrote:

I think that implementing PEP 422 as part of the language only makes sense if we once would be able to drop metaclasses altogether. I thought about adding a newclass to PEP 422 that would simulate the new in metaclasses, thinking that this is the only way metaclasses are used. Well, if you want Python to drop metaclasses, the way starts with PEP 422. You have to introduce the alternative first, and then wait a really long time until you drop a feature. I think newclass can be added after PEP 422 is in, if it turns out to be necessary.

There's nothing wrong with metaclasses per se - they work fine for implementing things like abc.ABC, enum.Enum, SQL ORMs and more. There's just a significant subset of their current use cases that doesn't need their full power and could benefit from having a simpler alternative mechanism available.

PEP 422 aims to carve out that subset and provide a simpler way of doing the same thing. A pure Python equivalent has existed in Zope for over a decade (which is referenced from the PEP after someone pointed it out in one of the earlier review rounds).

If someone had the time to update the PEP to address the issues raised in the last round of reviews, I'd be happy to add a third co-author :)

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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