Why does datetime.timedelta only have the attributes 'days' and 'seconds'?

[Chris Angelico]

On Sun, 17 Apr 2022 at 03:52, Peter J. Holzer <hjp-python at hjp.at> wrote:
>
> On 2022-04-17 02:46:38 +1000, Chris Angelico wrote:
> > On Sun, 17 Apr 2022 at 02:45, Peter J. Holzer <hjp-python at hjp.at> wrote:
> > > On 2022-04-17 02:14:44 +1000, Chris Angelico wrote:
> > > > So which one is it? Which one do you get when you add days=7 to a datetime?
> > >
> > > For adding a datetime and timedelta I think the answer is clear.
> > > But subtracting two datetimes is ambiguous.
> > >
> >
> > But if the difference between two datetimes is a timedelta, then
> > surely adding a timedelta to a datetime should give the other
> > datetime?
>
> Not necessarily. You might compute the difference for another purpose.
> If you compute a change rate from two gauge readings you would compute
> something like (r1 - r0) / (t1 - t0). You don't intend to add (t1 - t0)
> to any timestamp, so that property would be irrelevant. However, you do
> want something which can be used in a division and which has a
> consistent unit (so one could argue that you don't want a timedelta
> object at all, but a floating point number).
>

True, but logically, it's hard to explain arithmetic when (x - y) + x
!= y. ( And yes, I'm aware that floats can violate that, but the
discrepancy isn't a good thing.)

Your example definitely wants to be measured in UTC, though. It wants
to ignore silly changes of clocks, and just show the amount of time
that passed.

ChrisA