[Python-Dev] PEP 487 vs 422 (dynamic class decoration) (original) (raw)

PJ Eby pje at telecommunity.com
Mon Apr 6 01:17:06 CEST 2015

On Sat, Apr 4, 2015 at 9:33 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

So actually reading https://gist.github.com/pjeby/75ca26f8d2a7a0c68e30 properly, you're starting to convince me that a "noconflict" metaclass resolver would be a valuable and viable addition to the Python 3 type system machinery.

The future possible language level enhancement would then be to make that automatic resolution of metaclass conflicts part of the default metaclass determination process. I realise you've been trying to explain that to me for a few days now, I'm just writing it out explicitly to make it clear I finally get it :)

I'm glad you got around to reading it. Sometimes it's really frustrating trying to get things like that across.

What's funny is that once I actually 1) wrote that version, and 2) ended up doing a version of six's with_metaclass() function so I could write 2/3 mixed code in DecoratorTools, I realized that there isn't actually any reason why I can't write a Python 2 version of noconflict. Indeed, with a slight change to eliminate ClassType from the metaclass candidate list, the Python 3 version would also work as the Python 2 version: just use it as the explicit metaclass, or use with_metaclass, i.e.:

class something(base1, base2, ...):
    __metaclass__ = noconflict

    # ...

or:

class something(with_metaclass(noconflict, base1, base2, ...)):
    # ...

And the latter works syntactically from Python 2.3 on up.

My apologies for that - while I don't actually recall what I was thinking when I said it, I suspect I was all fired up that PEP 422 was definitely the right answer, and hence thought I'd have an official solution in place for you in fairly short order. I should have let you know explicitly when I started having doubts about it, so you could reassess your porting options.

Well, at least it's done now. Clearing up the issue allowed me to spend some time on porting some of the relevant libraries this weekend, where I promptly ran into challenges with several of the other features removed from Python 3 (like tuple arguments), but fortunately those are issues more of syntactic convenience than irreplaceable functionality. ;-)

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