[Datetime-SIG] Another approach to 495's glitches

Alexander Belopolsky alexander.belopolsky at gmail.com
Sun Sep 6 22:53:41 CEST 2015

On Sun, Sep 6, 2015 at 2:58 PM, Tim Peters <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.
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