[Datetime-SIG] PEP 495: What's left to resolve

[Chris Angelico]

On Wed, Sep 9, 2015 at 12:10 PM, Tim Peters <tim.peters at gmail.com> wrote:
> [Alex]
>>> Definitions:  An aware datetime value t is called "regular" if t.utcoffset()
>>> does not depend on the value of the fold attribute.
>
> [Chris Angelico]
>> One point to clarify here. Is the definition of "regular" based on the
>> timezone alone (that is to say, a UTC datetime is regular, and an
>> Australia/Brisbane datetime is regular, but anything in a region with
>> DST is always special), or are "special" datetimes only those in the
>> fold period?
>
> It applies to "an aware datetime value t".  That's clear already ;-)
> Everything about `t` matters.  In plain English `t` is "regular" if
> and only if `t` is in neither a fold nor a gap.  So, e.g., all `t` in
> UTC are regular.  In most zones with a notion of DST, there are
> exactly 2 wall-clock hours per year that are not regular (in the gap
> at the start of DST, and in the fold at DST end).

Okay, that's what I thought it meant. And it's easy enough to see if
two datetimes differ only in fold. The problem I was seeing was a
difficulty in recognizing whether a single datetime is special or not,
which is answered here:

On Wed, Sep 9, 2015 at 12:02 PM, Alexander Belopolsky
<alexander.belopolsky at gmail.com> wrote:
> If you want to know whether t is regular you have to compare t.utcoffset()
> and t.replace(fold=1-t.fold).utcoffset().  If they are the same, t is
> regular.  If not - t is special.

Thanks Alex!

(I can imagine pushing this to the timezone object as a primitive,
which will allow it to be optimized down to "t is regular" for
timezones that are always regular, but that's an optimization only.)

ChrisA