[Datetime-SIG] Calendar vs timespan calculations...

[Chris Barker]

On Thu, Jul 30, 2015 at 9:45 AM, Tim Peters <tim.peters at gmail.com> wrote:

> Oops!  Forgot one:
>
> [Guido]
> > ... create new timedelta-ish classes that implement "human" arithmetic
>
> Note that there's an annoying inelegance here:  after
>
>      datetime2 = datetime1 + timedeltaish  # or subtraction
>
> then what does:
>
>      datetime2 - datetime1
>
> produce?


yup -- with two (or three!) different flavors of a delta, what can you do?


> Of course that's never a problem today.  If the inelegance is thought
> to be unbearable, I suppose new timedelta-ish classes could provide an
> explicit class method to subtract datetime instances (yielding "the
> right" timedelta-ish object of the class).
>

Which makes me think -- is it important that there is ONE datetime object,
with two delta objects? or should there be two timedelta objects? That
might provide a more obvious API -- which kind of arithmetic do you want to
do with these? But also, it would open the door to different
implementations: for instance, if your primary goal is to support Duration
arithmetic, it may make sense to store the datetime in UTC always, and only
mess with time zones on I/O. And you may want to store as "microseconds
since year 1" directly  as well.

Of course, easy conversion between the two would be critical.

-Chris


-- 

Christopher Barker, Ph.D.
Oceanographer

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