Backport PR #36050: BUG: incorrect year returned in isocalendar for certain dates (#36127)

Co-authored-by: Asish Mahapatra <asishkm@gmail.com>

Author: meeseeksmachine

doc/source/whatsnew/v1.1.2.rst

@@ -33,6 +33,7 @@ Bug fixes
 - Bug in :meth:`DataFrame.apply` with ``result_type="reduce"`` returning with incorrect index (:issue:`35683`)
 - Bug in :meth:`DateTimeIndex.format` and :meth:`PeriodIndex.format` with ``name=True`` setting the first item to ``"None"`` where it should be ``""`` (:issue:`35712`)
 - Bug in :meth:`Float64Index.__contains__` incorrectly raising ``TypeError`` instead of returning ``False`` (:issue:`35788`)
+- Bug in :meth:`Series.dt.isocalendar` and :meth:`DatetimeIndex.isocalendar` that returned incorrect year for certain dates (:issue:`36032`)
 - Bug in :class:`DataFrame` indexing returning an incorrect :class:`Series` in some cases when the series has been altered and a cache not invalidated (:issue:`33675`)
 
 .. ---------------------------------------------------------------------------

pandas/_libs/tslibs/ccalendar.pyx

@@ -201,10 +201,10 @@ cpdef iso_calendar_t get_iso_calendar(int year, int month, int day) nogil:
             iso_week = 1
 
     iso_year = year
-    if iso_week == 1 and doy > 7:
+    if iso_week == 1 and month == 12:
         iso_year += 1
 
-    elif iso_week >= 52 and doy < 7:
+    elif iso_week >= 52 and month == 1:
         iso_year -= 1
 
     return iso_year, iso_week, dow + 1

pandas/tests/series/test_datetime_values.py

@@ -682,6 +682,9 @@ def test_setitem_with_different_tz(self):
             [[pd.NaT], [[np.NaN, np.NaN, np.NaN]]],
             [["2019-12-31", "2019-12-29"], [[2020, 1, 2], [2019, 52, 7]]],
             [["2010-01-01", pd.NaT], [[2009, 53, 5], [np.NaN, np.NaN, np.NaN]]],
+            # see GH#36032
+            [["2016-01-08", "2016-01-04"], [[2016, 1, 5], [2016, 1, 1]]],
+            [["2016-01-07", "2016-01-01"], [[2016, 1, 4], [2015, 53, 5]]],
         ],
     )
     def test_isocalendar(self, input_series, expected_output):

pandas/tests/tslibs/test_ccalendar.py

@@ -1,10 +1,13 @@
 from datetime import date, datetime
 
+from hypothesis import given, strategies as st
 import numpy as np
 import pytest
 
 from pandas._libs.tslibs import ccalendar
 
+import pandas as pd
+
 
 @pytest.mark.parametrize(
     "date_tuple,expected",
@@ -48,3 +51,15 @@ def test_dt_correct_iso_8601_year_week_and_day(input_date_tuple, expected_iso_tu
     expected_from_date_isocalendar = date(*input_date_tuple).isocalendar()
     assert result == expected_from_date_isocalendar
     assert result == expected_iso_tuple
+
+
+@given(
+    st.datetimes(
+        min_value=pd.Timestamp.min.to_pydatetime(warn=False),
+        max_value=pd.Timestamp.max.to_pydatetime(warn=False),
+    )
+)
+def test_isocalendar(dt):
+    expected = dt.isocalendar()
+    result = ccalendar.get_iso_calendar(dt.year, dt.month, dt.day)
+    assert result == expected