[Datetime-SIG] Another round on error-checking

Carl Meyer carl at oddbird.net
Thu Sep 3 17:37:11 CEST 2015

>> Currently you only get results that violate arithmetic identities if you
>> mix arithmetic within a timezone and arithmetic between timezones.
> And we currently have timeline conversions that can violate basic
> identities in _that_ space.  It is trading one for the other.

Yes. The new proposed behavior for PEP 495 abandons the assertion that
it can be "independent of arithmetic", recognizing that instead we're
trading consistency of arithmetic within a timezone for consistency of
round-trips between timezones.

So PEP 495 is already breaking the design of datetime, that tz-annotated
datetimes operate internally on a naive time model. It _has_ to break
that design, because it must introduce times that don't exist in that
model. But it's choosing to change that design piecemeal and
inconsistently instead of thoroughly and consistently.


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