[Datetime-SIG] Another round on error-checking

[Carl Meyer]

[Tim]
>>> There are still questions, like, e.g., what
>>>
>>>     fold_aware_datetime + timedelta
>>>
>>> should do when fold=1, but only in my variation of what you proposed.
>>> You proposed mixing "pay attention to fold" with "timeline
>>> arithmetic", which leaves no choice.  Alex and I seem to disagree
>>> about what to do when "only pay attention to fold" is meant instead.
[Alex]
>> This is one of those cases where I don't have a strong opinion.
[Tim]
> I do:  it should ignore fold=1.  Precisely the opposite of what you
> _thought_ I've been saying ;-)

[Alex]
>> Unlike the datetime - datetime case where we have a strong argument
>> to do timeline arithmetic in the presence of fold=1 (namely to preserve
>> the hash invariant),
> 
> And total ordering, and equivalence between comparison outcomes and
> subtraction results.  There are any number of "common sense"
> invariants that rely on this.

IIUC, choosing this combination of behavior means that it is possible to
have a datetime `dt1` (with fold=1) such that:

dt1 - dt2 => delta

where `fold` is respected in this case, but

dt2 + delta != dt1

because fold is ignored for timedelta arithmetic (but is respected for
equality-checking, because that's necessary to maintain the hashing
invariant).

Are we really so wedded to maintaining an unpredictable hybrid
naive/aware model for timezone-annotated datetimes that we're willing to
break basic invariants of arithmetic and equality to preserve it?

>> any choice here will lead to surprises.
> 
> Indeed so.  So screw it ;-)

An alternative to "so screw it" in the face of this puzzle would be to
choose the option that preserves all the invariants and behaves
predictably in all cases. But I suppose that would make things too easy...

Carl

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