[Python-Dev] datetime module enhancements

Steven Bethard steven.bethard at gmail.com
Fri Mar 9 23:03:02 CET 2007


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.)

STeVe
-- 
I'm not *in*-sane. Indeed, I am so far *out* of sane that you appear a
tiny blip on the distant coast of sanity.
        --- Bucky Katt, Get Fuzzy


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