Author veky
Recipients barry, eli.bendersky, ethan.furman, ezio.melotti, martin.panter, r.david.murray, serhiy.storchaka, veky
Date 2016-08-15.07:52:44
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Message-id <1471247565.61.0.21734623575.issue23591@psf.upfronthosting.co.za>
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It's true, but it seems that with Enums, we're trying to retrain people to use identity testing (see https://docs.python.org/3/library/enum.html#comparisons). It would be unfortunate if we had to reuntrain them. :-/

Ethan's proposal (of caching weakrefs) is fine, if he manages to do it correctly (I didn't). It's much more complicated than it seems at first, since it has to work during the class definition itself, too.

Ok, second problem I encountered: zeros. Imagine

    class MyFlags(enum.Flags):
        NONE = 0
        FIRST = 1 << 0
        SECOND = 1 << 1

What is MyFlags.FIRST & MyFlags.SECOND? 0 or MyFlags.NONE? Or even False? :-) I would almost be for the third option, if not for the fact that & is used not only for "membership" testing, but also (with ~) for clearing flags. Imitating Go, we might want to introduce another operator, &~, for clearing flags, but that would probably be a too great disruption.

MyFlags.NONE seems cool, but then we have an enum member that is false, another thing we have tried to avoid with current Enum design (that's why functional API starts values at 1, for example). And what about the case when MyFlags doesn't define NONE, only FIRST and SECOND?

If you opt for 0, you open another can of worms: first, return type depends on argument values, and Guido hates that. :-) Also, that 0 is still somehow tied to MyFlags: it would be a type error to write (MyFlags.FIRST & MyFlags.SECOND) | AnotherFlags.SOMETHING, right?

Maybe the correct solution is to _always_ have a special value in every Flags class for such cases, but then what is it's ~__NONE__? :-) Fortunately, once you add __NONE__ and __ALL__, you get a Boolean closure, and you can meaningfully define &, |, ^ and ~ in a type-safe way. But that's probably overkill.

And so on... there are a lot of problems, and I've been through them. I'd like you to succeed where I didn't, but I have a bad feeling.
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2016-08-15 07:52:45vekysetrecipients: + veky, barry, ezio.melotti, r.david.murray, eli.bendersky, ethan.furman, martin.panter, serhiy.storchaka
2016-08-15 07:52:45vekysetmessageid: <1471247565.61.0.21734623575.issue23591@psf.upfronthosting.co.za>
2016-08-15 07:52:45vekylinkissue23591 messages
2016-08-15 07:52:44vekycreate