From 6e7de8670c7375c71e5a37e31467fc0f4c3cbe7f Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Thu, 25 Jun 2026 11:44:05 +0200
Subject: [PATCH] [3.14] gh-152079: Fix `_datetime.fromisoformat()` mishandling
 a sub-second tz offset (GH-152087) (#152175)
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(cherry picked from commit 6f9c76d8d86997012acfa09fed05396aa9349bbf)

Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com>
Co-authored-by: Stan Ulbrych <stan@python.org>
---
 Lib/test/datetimetester.py                    | 26 +++++++++++++++++++
 ...-06-24-12-00-00.gh-issue-152079.f1tzus.rst |  3 +++
 Modules/_datetimemodule.c                     |  4 +--
 3 files changed, 31 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152079.f1tzus.rst

diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 80891903a5e5..b8474b16637d 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -3626,6 +3626,32 @@ class TestDateTime(TestDate):
 
         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
diff --git a/Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152079.f1tzus.rst b/Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152079.f1tzus.rst
new file mode 100644
index 000000000000..492d00724f6a
--- /dev/null
+++ b/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.
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index b4a93819b242..3122d0196519 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -1660,8 +1660,8 @@ 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) {
+        // Create a timezone from the offset (a zero offset returns UTC)
+        if (tzoffset == 0 && tz_useconds == 0) {
             return Py_NewRef(CONST_UTC(NO_STATE));
         }
 
-- 
2.47.3