[Datetime-SIG] Another approach to 495's glitches
Carl Meyer
carl at oddbird.net
Mon Sep 7 02:36:51 CEST 2015
On 09/06/2015 02:53 PM, Alexander Belopolsky wrote:
> On Sun, Sep 6, 2015 at 2:58 PM, Tim Peters <tim.peters at gmail.com
> <mailto:tim.peters at gmail.com>> wrote:
>
> [Tim]
> > ...
> > Consider two aware datetimes that compare equal. The task is to prove
> > they have the same hash. The subtlety is that while __eq__ and
> > __hash__ both use a notion of "UTC equivalent", they're not always the
> > same notion. __eq__ always uses the given values of `fold`, while
> > __hash__ always forces fold=0.
>
> Which obviously ;-) suggests yet another, possibly cleaner, approach:
> have interzone subtraction, and all interzone comparisons, _also_
> force fold to 0 (instead of having only interzone __eq__ and __ne__
> special-case fold=1) .
>
> I would not go that far. While interzone subtraction between arbitrary
> zones is a rarely needed overkill, I find it useful to have subtraction
> work between a local zone and UTC. For me, subtraction in this case is
> similar to conversion. Fix the EPOCH and d = t - EPOCH together with t
> = EPOCH + d gives you a bijection between times and timedeltas. From
> that, you are one step away from various numeric time scales. For
> example (t - datetime(1, 1, 1, tzinfo=timezone.utc)) //
> timedelta.resolution will give you a bijection between datetimes and
> some range of integers. Thus if we are going to "sell" fold as a way to
> implement conversions that "always work", I think we should include
> these types of conversions as well.
FWIW, Tim's latest proposal (either variant) resolves all my concerns
with PEP 495 (as I explained at greater length in the "Timeline
arithmetic" thread).
Fundamentally I don't care between these two variants (because the
difference between them only impacts interzone operations, and my
general advice on those going forward would be "don't use them").
Carl
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