[New-bugs-announce] [issue32404] fromtimestamp does not call __new__ in datetime subclasses

[Paul Ganssle]

New submission from Paul Ganssle <p.ganssle at gmail.com>:

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).

----------
messages: 308908
nosy: belopolsky, p-ganssle
priority: normal
severity: normal
status: open
title: fromtimestamp does not call __new__ in datetime subclasses
versions: Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32404>
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