New submission from Paul Ganssle <p.gans...@gmail.com>:
When preparing some tests for how subclasses of date and datetime react as part
of a fix for issue 32403, I noticed a fairly big example of where subclass is
not preserved - `tzinfo.fromutc`:
from datetime import datetime, timezone
class DateTimeSubclass(datetime):
pass
dt = DateTimeSubclass(2012, 1, 1)
dt2 = dt.astimezone(timezone.utc)
print(type(dt))
print(type(dt2))
# Result:
# <class '__main__.DateTimeSubclass'>
# <class 'datetime.datetime'>
This also affects `datetime.fromtimestamp` and `datetime.now`, since both of
these, when passed a time zone argument, will call `fromutc` internally. I
personally think that Python's `tzinfo.fromutc` should preserve the object's
class, but this *is* counter to the current API.
And either way, it's quite inconsistent to have `DateTimeSubclass.now()` return
`DateTimeSubclass` but have `DateTimeSubclass.now(timezone.utc)` return
`datetime.datetime`.
There is probably a somewhat inelegant way to get the alternate constructors
working properly (ignore the type of the argument up until the last return and
then construct the subclass from the components of the datetime), but I think
it might be better to fix the behavior of tzinfo.fromutc.
Somewhat related to issue 32404 and 31222, in that this concerns which
operations preserve type in subclasses.
----------
messages: 308961
nosy: belopolsky, p-ganssle
priority: normal
severity: normal
status: open
title: fromutc does not respect datetime subclasses
versions: Python 3.5, Python 3.6, Python 3.7, Python 3.8
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue32417>
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