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

Tim Peters tim.peters at gmail.com
Sun Sep 13 03:58:48 CEST 2015

>> Those pytz methods work for any (pytz) timezone -- astimezone() with a
>> default argument only works for the local time zone.

> That's what os.environ['TZ'] = zonename is for.  The  astimezone() method
> works for every timezone installed on your system.  Try it - you won't even
> need to call time.tzset()!

I tried it.  It makes no difference to anything for me.  I stay on
Windows to remind people that millions of Python users don't see any
of the horrid nonsense Linuxish systems force on poor users ;-)

> ...
> In any case, there are three approaches to designing a TZ database interface
> in the datetime module: the "as intended" approach, the pytz approach and
> the astimezone(zonename:str) approach.

Portability rules out #3, unless Python bundles its own zoneinfo wrapping.

pytk's approach has many attractions, like no need for `fold` and no
breakage of anything, and blazing fast .utcoffset().   Except at least
<datetime, timedelta> arithmetic would have to be patched to do a
`normalize` variant by magic (to attach the now-appropriate
fixed-offset tzinfo, but without changing the clock in the process).
Alas, that would be a huge speed hit for classic arithmetic.

So, as always, the original intent is the only one that makes sense in
the end ;-)

> ...
> That's why I believe PEP 495 followed by the implementation
> of fold-aware "as intended" tzinfos (either within stdlib or by third
> parties) is the right approach.

Me too - except I think acceptance of 495 should be contingent upon
someone first completing a fully functional (if not releasable)
fold-aware zoneinfo wrapping.  Details have a way of surprising, and
we should learn from the last time we released a tzinfo spec in the
absence of any industrial-strength wrappings using it.

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