[3.15] gh-159706: Fix pure-Python ZoneInfo.fromutc() fold detection for single-transition zones (GH-159828) (#160152)

(cherry picked from commit 1c5b959)

Co-authored-by: Lazizbek Ergashev <lazerg2@gmail.com>

Author: miss-islington

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -642,6 +642,26 @@ def test_one_transition(self):
                 dt_utc_actual = dt_local.astimezone(timezone.utc)
                 self.assertEqual(dt_utc_actual, dt_utc)
 
+    def test_one_transition_fold(self):
+        GMT = ZoneOffset("-00", ZERO)
+        N03 = ZoneOffset("-03", -timedelta(hours=3))
+
+        transitions = [
+            ZoneTransition(datetime(1976, 12, 1), GMT, N03),
+        ]
+
+        after = "<-03>3"
+
+        zf = self.construct_zone(transitions, after)
+        zi = self.klass.from_file(zf)
+
+        dt_utc = datetime(1976, 12, 1, 1, tzinfo=timezone.utc)
+        dt_local = dt_utc.astimezone(zi)
+
+        self.assertEqual(dt_local.fold, 1)
+        self.assertEqual(dt_local.utcoffset(), N03.utcoffset)
+        self.assertEqual(dt_local.astimezone(timezone.utc), dt_utc)
+
     def test_one_zone_dst(self):
         DST = ZoneOffset("DST", ONE_H, ONE_H)
         transitions = [

Lib/zoneinfo/_zoneinfo.py

@@ -144,7 +144,7 @@ def fromutc(self, dt):
             if num_trans > 1 and timestamp >= self._trans_utc[1]:
                 tti_prev, tti = self._ttinfos[idx - 2 : idx]
             elif timestamp > self._trans_utc[-1]:
-                tti_prev = self._ttinfos[-1]
+                tti_prev = self._tti_before
                 tti = self._tz_after
             else:
                 tti_prev = self._tti_before

Misc/NEWS.d/next/Library/2026-10-19-16-49-09.gh-issue-159706.ushGrx.rst

@@ -0,0 +1,3 @@
+Fix the pure-Python :class:`zoneinfo.ZoneInfo` setting ``fold=0`` in
+:meth:`~datetime.tzinfo.fromutc` for times in the fold created by the only
+transition of a zone with a single transition.