From 0b07ea6b98e5bd9e445e87892da25108977a7488 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?tonghuaroot=20=28=E7=AB=A5=E8=AF=9D=29?=
 <tonghuaroot@gmail.com>
Date: Thu, 25 Jun 2026 17:15:28 +0800
Subject: [PATCH] gh-152079: Fix `_datetime.fromisoformat()` mishandling a
 sub-second tz offset (GH-152087) (cherry picked from commit
 6f9c76d8d86997012acfa09fed05396aa9349bbf)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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 6749fad35a64d9..3e7aebe8ecf9bf 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -3416,6 +3416,32 @@ 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
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 00000000000000..492d00724f6a46
--- /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 8df5913dfaee3d..e0d1f57c259c42 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -1637,8 +1637,8 @@ 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));
         }