[Python-Dev] datetime module enhancements

[Collin Winter]

On 3/9/07, Steven Bethard <steven.bethard at gmail.com> wrote:
> On 3/9/07, Collin Winter <collinw at gmail.com> wrote:
> > One solution that just occurred to me -- and that skirts the issue of
> > choosing an interpretation -- is that, when comparing date and
> > datetime objects, the datetime's .date() method is called and the
> > result of that call is compared to the original date. That is,
> >
> > datetime_obj < date_obj
> >
> > is implicitly equivalent to
> >
> > datetime_obj.date() < date_obj
>
> Using the .date() is fine when the year/month/day doesn't match.  So
> the following are fine::
>     datetime.datetime(2005, 1, 1, 0, 0, 0) < datetime.date(2006, 1, 1)
>     datetime.datetime(2007, 1, 1, 0, 0, 0) > datetime.date(2006, 1, 1)
> It's *not* okay to say that a date() is less than, greater than or
> equal to a datetime() if the year/month/day *does* match.  The correct
> temporal relation is During, but Python doesn't have a During
> operator. During is not the same as less-than, greater-than or
> equal-to, so all of these should be False::
>     datetime.datetime(2006, 1, 1, 0, 0, 0)  < datetime.date(2006, 1, 1)
>     datetime.datetime(2006, 1, 1, 0, 0, 0)  > datetime.date(2006, 1, 1)
>     datetime.datetime(2006, 1, 1, 0, 0, 0)  == datetime.date(2006, 1, 1)
> That is, the datetime() is not less than, greater than or equal to the
> corresponding date().
>
> Some discussion of these kinds of issues is here:
>     http://citeseer.ist.psu.edu/allen94actions.html
> The essence is that in order to properly compare intervals, you need
> the Meets, Overlaps, Starts, During and Finishes operators in addition
> to the Before (<) and Simulaneous (=) operators.
>
> So, let's not conflate Before, After or Simultaneous with the other
> relations -- if it's not strictly Before (<), After (>) or
> Simultaneous (=), we can just say so by returning False.
>
> (I believe these semantics would be just fine for both of the examples
> offered so far, but let me know if you think that's not true.)

I can't say I'm well-versed in the intricacies of date/time issues,
but what you say makes sense. This is exactly why I brought this patch
up here : )

Thanks,
Collin Winter