datetime.time.fromisoformat loses subsecond-only UTC offsets

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Python datetime documentation, time.fromisoformat: "Return a time corresponding to a time_string in any valid ISO 8601 format, with the following exceptions:" and "Time zone offsets may have fractional seconds." The time.isoformat documentation specifies that UTC offsets can be emitted as '+HH:MM:SS.ffffff'.

Expected: datetime.timedelta(microseconds=1)

Actual: datetime.timedelta(0)

import datetime as d

try:
    expected = d.timedelta(microseconds=1)
    t = d.time(12, 34, 56, tzinfo=d.timezone(expected))
    s = t.isoformat()
    if s != "12:34:56+00:00:00.000001":
        print("REFUTATION REJECTED: reported input was not reproduced")
    else:
        # Valid time and offset, in documented +HH:MM:SS.ffffff form.
        actual = d.time.fromisoformat(s).utcoffset()
        if actual != expected:
            print(f"REFUTATION CONFIRMED: {s!r} actual={actual!r} expected={expected!r}")
        else:
            print("REFUTATION REJECTED: actual equals documented expectation")
except Exception as e:
    print(f"REFUTATION REJECTED: {type(e).__name__}: {e}")

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

REFUTATION CONFIRMED: '12:34:56+00:00:00.000001' actual=datetime.timedelta(0) expected=datetime.timedelta(microseconds=1)

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/datetime-20261003-014717-c2

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).