In writing some tests for the alternate date constructors as part of my PR for issue 32403 (https://bugs.python.org/issue32403), I noticed that for `datetime`, the `fromtimestamp` bypasses the `__new__` call on the subclass: from datetime import datetime args = (2003, 4, 14) ts = 1050292800.0 # Equivalent timestamp d_ord = 731319 # Equivalent ordinal date class DatetimeSubclass(datetime): def __new__(cls, *args, **kwargs): result = datetime.__new__(cls, *args, **kwargs) result.extra = 7 return result base_d = DatetimeSubclass(*args) assert isinstance(base_d, DatetimeSubclass) # Passes assert base_d.extra == 7 # Passes ord_d = DatetimeSubclass.fromordinal(d_ord) assert isinstance(ord_d, DatetimeSubclass) # Passes assert ord_d.extra == 7 # Passes ts_d = DatetimeSubclass.fromtimestamp(ts) assert isinstance(ts_d, DatetimeSubclass) # Passes assert ts_d.extra == 7 # Fails Replacing `datetime` with `date` in the above code we don't get a failure, but with `datetime`, it fails with: AttributeError: 'DatetimeSubclass' object has no attribute 'extra' Regardless of the status of 32403, I think this should be fixed (though I can try to fix them both at the same time).

 New changeset 9f1b7b93f5f0ef589e7b272e127cacf4ce5d23f1 by Alexander Belopolsky (Paul Ganssle) in branch 'master': bpo-32403: Faster date and datetime constructors (#4993) https://github.com/python/cpython/commit/9f1b7b93f5f0ef589e7b272e127cacf4ce5d23f1