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gh-152079: Fix C datetime.fromisoformat() dropping sub-second UTC offset #152087

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gh-152079: Fix C datetime.fromisoformat() dropping sub-second UTC offset #152087

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26 changes: 26 additions & 0 deletions Lib/test/datetimetester.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3802,6 +3802,32 @@ def test_fromisoformat_utc(self):

self.assertIs(dt.tzinfo, timezone.utc)

def test_fromisoformat_utc_subsecond_offset(self):
# A UTC offset whose whole-second part is zero but with a non-zero
# microsecond part must be preserved, not collapsed to UTC.
for us in (1, -1, 999999, -999999):
with self.subTest(microseconds=us):
tz = timezone(timedelta(microseconds=us))
dt = self.theclass(2020, 6, 15, 12, 34, 56, tzinfo=tz)
rt = self.theclass.fromisoformat(dt.isoformat())
self.assertEqual(rt.utcoffset(), timedelta(microseconds=us))
self.assertEqual(rt, dt)
self.assertIsNot(rt.tzinfo, timezone.utc)

tz = timezone(timedelta(hours=5, minutes=30, seconds=15,
microseconds=123456))
dt = self.theclass(2020, 6, 15, 12, 34, 56, tzinfo=tz)
rt = self.theclass.fromisoformat(dt.isoformat())
self.assertEqual(rt.utcoffset(), tz.utcoffset(None))
self.assertEqual(rt, dt)

for tstr in ('2020-06-15T12:34:56+00:00',
'2020-06-15T12:34:56+00:00:00.000000',
'2020-06-15T12:34:56Z'):
with self.subTest(tstr=tstr):
self.assertIs(self.theclass.fromisoformat(tstr).tzinfo,
timezone.utc)

def test_fromisoformat_subclass(self):
class DateTimeSubclass(self.theclass):
pass
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152079.f1tzus.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix :meth:`datetime.datetime.fromisoformat` in the C implementation dropping
the sub-second part of a UTC offset whose whole-second part is zero, matching
the pure-Python implementation.
2 changes: 1 addition & 1 deletion Modules/_datetimemodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1669,7 +1669,7 @@ tzinfo_from_isoformat_results(int rv, int tzoffset, int tz_useconds)
PyObject *tzinfo;
if (rv == 1) {
// Create a timezone from offset in seconds (0 returns UTC)
if (tzoffset == 0) {
if (tzoffset == 0 && tz_useconds == 0) {
return Py_NewRef(CONST_UTC(NO_STATE));
}

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