datetime.fromisoformat loses fractional UTC offsets smaller than one second

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Python datetime documentation, datetime.fromisoformat(): "Return a datetime corresponding to a date_string in any valid ISO 8601 format, with the following exceptions:" followed by "Time zone offsets may have fractional seconds." datetime.isoformat() documents its aware output as "YYYY-MM-DDTHH:MM:SS.ffffff+HH:MM[:SS[.ffffff]]".

Expected: Parsing '2024-01-02T12:00:00+00:00:00.000001' preserves utcoffset() == timedelta(microseconds=1).

Actual: Parsing returns a datetime with a zero UTC offset.

import datetime as dt

try:
    offset = dt.timedelta(microseconds=1)
    d = dt.datetime(2024, 1, 2, 12, tzinfo=dt.timezone(offset))
    s = d.isoformat()
    if s != "2024-01-02T12:00:00+00:00:00.000001":
        print("REFUTATION REJECTED: input does not match documented isoformat output")
    else:
        expected = offset // dt.timedelta(microseconds=1)
        actual = dt.datetime.fromisoformat(s).utcoffset() // dt.timedelta(microseconds=1)
        if actual != expected:
            print(f"REFUTATION CONFIRMED: {s!r} actual: {actual} expected: {expected} (microseconds)")
        else:
            print("REFUTATION REJECTED: fractional-second offset preserved")
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: '2024-01-02T12:00:00+00:00:00.000001' actual: 0 expected: 1 (microseconds)

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-c4

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).