On Sun, Sep 6, 2015 at 2:58 PM, Tim Peters <[email protected]> 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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