From 278932b01e7e0641d7cf23f10125b04bfd4ea23a Mon Sep 17 00:00:00 2001
From: Vyas Ramasubramani <vyasr@nvidia.com>
Date: Sun, 6 Sep 2026 11:14:46 -0700
Subject: [PATCH] TEST: Target timezone DST edge cases

---
 python/cudf/cudf/tests/conftest.py            | 47 ++++++++++++++++++-
 .../cudf/tests/series/accessors/test_dt.py    | 24 ++++++----
 2 files changed, 62 insertions(+), 9 deletions(-)

diff --git a/python/cudf/cudf/tests/conftest.py b/python/cudf/cudf/tests/conftest.py
index 91ecb3247bf0..5a1009571159 100644
--- a/python/cudf/cudf/tests/conftest.py
+++ b/python/cudf/cudf/tests/conftest.py
@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: Copyright (c) 2019-2026, NVIDIA CORPORATION.
+# SPDX-FileCopyrightText: Copyright (c) 2019-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
 import datetime
@@ -98,6 +98,19 @@ def _get_all_zones():
     return sorted(zones)
 
 
+def _get_transition_zones(timestamps, always_include):
+    zones = set(always_include)
+    for zone in _get_all_zones():
+        timezone = zoneinfo.ZoneInfo(zone)
+        if any(
+            timestamp.replace(tzinfo=timezone, fold=0).utcoffset()
+            != timestamp.replace(tzinfo=timezone, fold=1).utcoffset()
+            for timestamp in timestamps
+        ):
+            zones.add(zone)
+    return sorted(zones)
+
+
 # NOTE: _get_all_zones is a very large list; we likely do NOT want to
 # use it for more than a handful of tests
 @pytest.fixture(params=_get_all_zones())
@@ -105,6 +118,38 @@ def all_timezones(request):
     return request.param
 
 
+@pytest.fixture(
+    params=_get_transition_zones(
+        [
+            datetime.datetime(2018, 11, 4, 0, 30),
+            datetime.datetime(2018, 11, 4, 1),
+            datetime.datetime(2018, 11, 4, 1, 30),
+            datetime.datetime(2018, 11, 4, 2),
+            datetime.datetime(2018, 11, 4, 2, 30),
+        ],
+        {"America/Metlakatla", "UTC"},
+    )
+)
+def ambiguous_timezones(request):
+    return request.param
+
+
+@pytest.fixture(
+    params=_get_transition_zones(
+        [
+            datetime.datetime(2018, 3, 11, 1, 30),
+            datetime.datetime(2018, 3, 11, 2),
+            datetime.datetime(2018, 3, 11, 2, 30),
+            datetime.datetime(2018, 3, 11, 3),
+            datetime.datetime(2018, 3, 11, 3, 30),
+        ],
+        {"America/Grand_Turk", "UTC"},
+    )
+)
+def nonexistent_timezones(request):
+    return request.param
+
+
 @pytest.fixture(
     params=["America/New_York", "Asia/Tokyo", "CET", "Etc/GMT+1", "UTC"]
 )
diff --git a/python/cudf/cudf/tests/series/accessors/test_dt.py b/python/cudf/cudf/tests/series/accessors/test_dt.py
index ba4fc2ffe22e..791e86f46a7a 100644
--- a/python/cudf/cudf/tests/series/accessors/test_dt.py
+++ b/python/cudf/cudf/tests/series/accessors/test_dt.py
@@ -753,10 +753,12 @@ def test_tz_localize(datetime_types_as_str, all_timezones):
     assert str(s.dtype.tz) == all_timezones
 
 
-def test_localize_ambiguous(request, datetime_types_as_str, all_timezones):
+def test_localize_ambiguous(
+    request, datetime_types_as_str, ambiguous_timezones
+):
     request.applymarker(
         pytest.mark.xfail(
-            condition=(all_timezones == "America/Metlakatla"),
+            condition=(ambiguous_timezones == "America/Metlakatla"),
             reason="https://www.timeanddate.com/news/time/metlakatla-quits-dst.html",
         )
     )
@@ -772,16 +774,20 @@ def test_localize_ambiguous(request, datetime_types_as_str, all_timezones):
         dtype=datetime_types_as_str,
     )
     expect = s.to_pandas().dt.tz_localize(
-        zoneinfo.ZoneInfo(all_timezones), ambiguous="NaT", nonexistent="NaT"
+        zoneinfo.ZoneInfo(ambiguous_timezones),
+        ambiguous="NaT",
+        nonexistent="NaT",
     )
-    got = s.dt.tz_localize(all_timezones)
+    got = s.dt.tz_localize(ambiguous_timezones)
     assert_eq(expect, got)
 
 
-def test_localize_nonexistent(request, datetime_types_as_str, all_timezones):
+def test_localize_nonexistent(
+    request, datetime_types_as_str, nonexistent_timezones
+):
     request.applymarker(
         pytest.mark.xfail(
-            condition=all_timezones == "America/Grand_Turk",
+            condition=nonexistent_timezones == "America/Grand_Turk",
             reason="https://www.worldtimezone.com/dst_news/dst_news_turkscaicos03.html",
         )
     )
@@ -797,9 +803,11 @@ def test_localize_nonexistent(request, datetime_types_as_str, all_timezones):
         dtype=datetime_types_as_str,
     )
     expect = s.to_pandas().dt.tz_localize(
-        zoneinfo.ZoneInfo(all_timezones), ambiguous="NaT", nonexistent="NaT"
+        zoneinfo.ZoneInfo(nonexistent_timezones),
+        ambiguous="NaT",
+        nonexistent="NaT",
     )
-    got = s.dt.tz_localize(all_timezones)
+    got = s.dt.tz_localize(nonexistent_timezones)
     assert_eq(expect, got)