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

The C accelerator's tzinfo_from_isoformat_results() collapsed any offset
with a zero whole-second part to timezone.utc, discarding the parsed
sub-second microseconds. So '+00:00:00.000001' round-tripped through
isoformat()/fromisoformat() lost its 1-microsecond offset, while pure
Python preserved it. Only short-circuit to UTC when both the
whole-second and sub-second parts are zero.

Author: tonghuaroot

Lib/test/datetimetester.py

@@ -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

Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152079.f1tzus.rst

@@ -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.

Modules/_datetimemodule.c

@@ -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));
         }