gh-152079: Fix _datetime.fromisoformat() mishandling a sub-second tz offset (#152087)

tonghuaroot · StanFromIreland · web-flow · commit 6f9c76d8d869 · 2026-06-25T10:15:28.000+01:00
Co-authored-by: Stan Ulbrych &lt;stan@python.org&gt;
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
@@ -3803,6 +3803,32 @@ 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
@@ -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
@@ -1668,8 +1668,8 @@ 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));
         }
 

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	+Fix :meth:`datetime.datetime.fromisoformat` in the C implementation dropping
	`2`	`+the sub-second part of a UTC offset whose whole-second part is zero, matching`
	`3`	`+the pure-Python implementation.`
Original file line number	Diff line number	Diff line change
`@@ -1668,8 +1668,8 @@ tzinfo_from_isoformat_results(int rv, int tzoffset, int tz_useconds)`
`1668`	`1668`	`{`
`1669`	`1669`	`PyObject *tzinfo;`
`1670`	`1670`	`if (rv == 1) {`
`1671`		`- // Create a timezone from offset in seconds (0 returns UTC)`
`1672`		`- if (tzoffset == 0) {`
	`1671`	`+ // Create a timezone from the offset (a zero offset returns UTC)`
	`1672`	`+ if (tzoffset == 0 && tz_useconds == 0) {`
`1673`	`1673`	`return Py_NewRef(CONST_UTC(NO_STATE));`
`1674`	`1674`	`}`
`1675`	`1675`