On Sun, 17 Apr 2022 at 03:52, Peter J. Holzer <hjp-pyt...@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-pyt...@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 -- https://mail.python.org/mailman/listinfo/python-list
