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

[Tin Tvrtković]

>
> Date: Fri, 13 Oct 2017 08:57:00 -0700
> From: Guido van Rossum <guido at python.org>
> To: Martin Teichmann <lkb.teichmann at gmail.com>
> Cc: Python-Dev <python-dev at python.org>
> Subject: Re: [Python-Dev] What is the design purpose of metaclasses vs
>         code generating decorators? (was Re: PEP 557: Data Classes)
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>         <
> CAP7+vJKBVuDqf09zTWDAuvQ-cCNM+cF82c22s2NJOj+A9k7_kA at mail.gmail.com>
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>
> This is food for thought. I'll have to let it sink in a bit, but you may be
> on to something.
>
> Since the question was asked at some point, yes, metaclasses are much older
> than class decorators. At some point I found the book Putting Metaclasses
> to Work by Ira Forman and Scott Danforth (
> https://www.amazon.com/Putting-Metaclasses-Work-Ira-Forman/dp/0201433052)
> and translated the book's ideas from C++ to Python, except for the
> automatic merging of multiple inherited metaclasses.
>
> But in many cases class decorators are more useful.
>
> I do worry that things like your autoslots decorator example might be
> problematic because they create a new class, throwing away a lot of work
> that was already done. But perhaps the right way to address this would be
> to move the decision about the instance layout to a later phase? (Not sure
> if that makes sense though.)
>
> --Guido
>

Just FYI, recreating the class with slots runs into problems with regards
to PEP 3135 (New Super). In attrs we resort to black magic to update the
__class__ cell in existing methods. Being able to add slotness later would
be good, but not that useful for us since we have to support down to 2.7.
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