I would like to call your attention to my PR #10902 making timedelta
addition respect the subclass of the object it was added to:
https://github.com/python/cpython/pull/10902
Because date and datetime have their classes hard-coded into their
__add__ methods, any datetime arithmetic /discards/ the subclass. Since
datetime arithmetic is a very common primitive in datetime operations,
this leads to all kinds of funky behaviors like the fact that
DatetimeSubclass.fromtimestamp(0) returns a DatetimeSubclass (as it
should), but DatetimeSubclass.fromtimestamp(0, timezone.utc) returns a
datetime.
I think that this is the right behavior (I have a more detailed
rationale here: https://bugs.python.org/issue35364#msg331065 ) and I
find it unlikely that there will be any significant bugs introduced by
this change except in very strange edge cases. I know that changing
return types like this can be seen as a major change (I do not, for
example, suggest backporting this before 3.8), so I am bringing it up
here to get some extra discussion on the matter.
Thanks,
Paul
