fromtimestamp does not call __new__ in datetime subclasses

[pganssle]

BPO	32404
Nosy	@abalkin, @pganssle
PRs	bpo-32403: Faster date and datetime constructors #4993

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-01-16.18:06:34.145>
created_at = <Date 2017-12-21.21:54:13.191>
labels = ['3.7', '3.8']
title = 'fromtimestamp does not call __new__ in datetime subclasses'
updated_at = <Date 2018-01-16.18:06:34.144>
user = 'https://github.com/pganssle'

bugs.python.org fields:

activity = <Date 2018-01-16.18:06:34.144>
actor = 'belopolsky'
assignee = 'none'
closed = True
closed_date = <Date 2018-01-16.18:06:34.145>
closer = 'belopolsky'
components = []
creation = <Date 2017-12-21.21:54:13.191>
creator = 'p-ganssle'
dependencies = []
files = []
hgrepos = []
issue_num = 32404
keywords = ['patch']
message_count = 2.0
messages = ['308908', '310100']
nosy_count = 2.0
nosy_names = ['belopolsky', 'p-ganssle']
pr_nums = ['4993']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue32404'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[pganssle]

In writing some tests for the alternate date constructors as part of my PR for bpo-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).

[abalkin]

New changeset 9f1b7b9 by Alexander Belopolsky (Paul Ganssle) in branch 'master':
bpo-32403: Faster date and datetime constructors (bpo-4993)
9f1b7b9