[Datetime-SIG] Are there any "correct" implementations of tzinfo?

Alexander Belopolsky alexander.belopolsky at gmail.com
Sat Sep 12 21:53:38 CEST 2015

On Sat, Sep 12, 2015 at 3:41 PM, Tim Peters <tim.peters at gmail.com> wrote:

> > If there are not, maybe the intended semantics should go
> > by the wayside and be replaced by what pytz does.
> Changing anything about default arithmetic behavior is not a
> possibility.  This has been beaten to death multiple times on this
> mailing list already, and I'm not volunteering for another round of it
> ;-)

Tim and Guido only grudgingly accept it, but datetime already gives you
"the pytz way" and PEP 495 makes a small improvement to it.  The
localize/normalize functionality is provided by the .astimezone() method
which when called without arguments will attach an appropriate fixed offset
timezone to a datetime object.   You can then add timedeltas to the result
and stay within a "fictitious" fixed offset timezone that extends
indefinitely in both directions.  To get back to the actual civil time -
you call .astimezone() again.  This gives you what we call here a
"timeline" arithmetic and occasionally it is preferable to doing arithmetic
in UTC.  (Effectively you do arithmetic in local standard time instead of
UTC.)   Using a fixed offset timezone other than UTC for timeline
arithmetic is preferable in timezones that are far enough from UTC that
business hours straddle UTC midnight.
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