[Python-Dev] Status on PEP-431 Timezones

Paul Moore p.f.moore at gmail.com
Mon Jul 27 18:45:40 CEST 2015


On 27 July 2015 at 17:30, Lennart Regebro <regebro at gmail.com> wrote:
>> Apologies for asking yet another dumb question about this, but I have
>> the impression that a lot of other people are struggling with the basics
>> here too.
>>
>> Can you tell us which of the two operations datetime currently
>> implements?
>
> It's intended that the first one is implemented, meaning that
> datetime.now() + timedelta(hours=24) can result in a datetime
> somewhere between 23 and 25 hours into the future. Or well, any
> amount, in theory, I guess some changes are more than an hour, but
> that's very unusual.

Maybe that's what the PEP intends. But the stdlib as currently
implemented simply adds the appropriate number to the relevant field
(i.e., "increase the hour count with one").

It's not possible to detect the difference between these two using
only stdlib timezones, though, as those are all fixed-offset.

Both Tim and I have pointed out that Guido's original intention was
precisely what is implemented, for better or worse. What Guido's view
was on DST-aware timezones and whether the behaviour was appropriate
for those, I don't know personally (maybe Tim does). It may well be
that it was "let's not think about it for now".

If we can ever straighten out what the question is, maybe Guido can
chip in and answer it :-)

Paul

PS Ideally, Guido could pop into his time machine and fix the whole
issue at source. But apparently he can't, because the time machine
isn't DST-aware...


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