plistlib binary serialization conflates aware datetimes with different fold-dependent UTC offsets

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The plistlib documentation for dump states: "When aware_datetime is true and the value is an aware datetime.datetime, it will be converted to UTC timezone before writing it." The load documentation states: "When aware_datetime is true, fields with type datetime.datetime will be created as aware datetime.datetime with datetime.UTC as tzinfo."

Expected: UTC datetimes 2020-06-15 12:00 and 2020-06-15 11:00.

Actual: Both decoded datetimes are 2020-06-15 12:00 UTC.

import datetime as d
import plistlib as p

class Zone(d.tzinfo):
    def utcoffset(self, dt):
        return d.timedelta(hours=dt.fold)
    def dst(self, dt):
        return d.timedelta(0)

try:
    z = Zone()
    values = [d.datetime(2020, 6, 15, 12, tzinfo=z, fold=f) for f in (0, 1)]
    offsets = [v.utcoffset() for v in values]
    if not (values[0].tzinfo is values[1].tzinfo
            and values[0].replace(tzinfo=None) == values[1].replace(tzinfo=None)
            and offsets == [d.timedelta(0), d.timedelta(hours=1)]):
        raise ValueError("input does not meet the reported conditions")
    expected = [(v.replace(tzinfo=None) - o).replace(tzinfo=d.timezone.utc)
                for v, o in zip(values, offsets)]
    actual = p.loads(p.dumps(values, fmt=p.FMT_BINARY, aware_datetime=True),
                     aware_datetime=True)
    if actual != expected:
        print("REFUTATION CONFIRMED:", "input:", values,
              "actual:", actual, "expected:", expected)
    else:
        print("REFUTATION REJECTED:", "actual matches the documented expectation")
except Exception as e:
    print("REFUTATION REJECTED:", type(e).__name__, str(e))

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library plistlib:

REFUTATION CONFIRMED: input: [datetime.datetime(2020, 6, 15, 12, 0, tzinfo=<__main__.Zone object at 0x000001F754246BA0>), datetime.datetime(2020, 6, 15, 12, 0, fold=1, tzinfo=<__main__.Zone object at 0x000001F754246BA0>)] actual: [datetime.datetime(2020, 6, 15, 12, 0, tzinfo=datetime.timezone.utc), datetime.datetime(2020, 6, 15, 12, 0, tzinfo=datetime.timezone.utc)] expected: [datetime.datetime(2020, 6, 15, 12, 0, tzinfo=datetime.timezone.utc), datetime.datetime(2020, 6, 15, 11, 0, tzinfo=datetime.timezone.utc)]

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/plistlib-20261003-073911-c4

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).