BUG: Return type discrepancy in USFederalHolidayCalendar (#49118)

* GH49075 looks weird.   Let's add a test and go from there.

* Some cleanup.

* First pass at the fix.

* Missed a space, failed formatting check.

* pre-commit would have caught that, eh.

* Update test_federal.py

* Well, that was subtle.

* Changed the test.

* Git mistake.

* Backing away from keyboard now.

* next: write more tests.

* precommit local run.

* User specified half-open date intervals can return inconsistent results.

* Added logic to close open time intervals into AbstractHolidayCalender

* OK, I've learned pre-commit doesn't pass wildcards all the way down a directory tree.

* Ensure DatetimeIndex returned in _apply_rule

* Ensure DatetimeIndex returned in _apply_rule

* Caught formatting.

* Missed running isort locally.

* Well, that was subtle.

* Changed URL to current OPM source-of-record, updated whatsnew

* Update v2.0.0.rst

* Add dtype to empty index ( caught by mypy in CI, not part of pre-commit )

* Updated holiday.py

* ... and forgot to pre-commit run black

* Update pandas/tseries/holiday.py

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

* Update v2.0.0.rst

* Added test to test_federal.py to ensure comparison against known-good result in addition to type comparison

* Add change to test_federal to compare against constructed DatetimeIndex, cleaned up test_half_open_interval_with_observance() in test_holiday.

* Update doc/source/whatsnew/v2.0.0.rst

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

Author: roadswitcher

doc/source/whatsnew/v2.0.0.rst

@@ -354,7 +354,7 @@ Datetimelike
 - Bug in :func:`to_datetime` was raising on invalid offsets with ``errors='coerce'`` and ``infer_datetime_format=True`` (:issue:`48633`)
 - Bug in :class:`DatetimeIndex` constructor failing to raise when ``tz=None`` is explicitly specified in conjunction with timezone-aware ``dtype`` or data (:issue:`48659`)
 - Bug in subtracting a ``datetime`` scalar from :class:`DatetimeIndex` failing to retain the original ``freq`` attribute (:issue:`48818`)
--
+- Bug in ``pandas.tseries.holiday.Holiday`` where a half-open date interval causes inconsistent return types from :meth:`USFederalHolidayCalendar.holidays` (:issue:`49075`)
 
 Timedelta
 ^^^^^^^^^

pandas/tests/tseries/holiday/test_federal.py

@@ -1,7 +1,11 @@
 from datetime import datetime
 
+from pandas import DatetimeIndex
+import pandas._testing as tm
+
 from pandas.tseries.holiday import (
     AbstractHolidayCalendar,
+    USFederalHolidayCalendar,
     USMartinLutherKingJr,
     USMemorialDay,
 )
@@ -36,3 +40,19 @@ class MemorialDay(AbstractHolidayCalendar):
         datetime(1978, 5, 29, 0, 0),
         datetime(1979, 5, 28, 0, 0),
     ]
+
+
+def test_federal_holiday_inconsistent_returntype():
+    # GH 49075 test case
+    # Instantiate two calendars to rule out _cache
+    cal1 = USFederalHolidayCalendar()
+    cal2 = USFederalHolidayCalendar()
+
+    results_2018 = cal1.holidays(start=datetime(2018, 8, 1), end=datetime(2018, 8, 31))
+    results_2019 = cal2.holidays(start=datetime(2019, 8, 1), end=datetime(2019, 8, 31))
+    expected_results = DatetimeIndex([], dtype="datetime64[ns]", freq=None)
+
+    # Check against expected results to ensure both date
+    # ranges generate expected results as per GH49075 submission
+    tm.assert_index_equal(results_2018, expected_results)
+    tm.assert_index_equal(results_2019, expected_results)

pandas/tests/tseries/holiday/test_holiday.py

@@ -3,6 +3,7 @@
 import pytest
 from pytz import utc
 
+from pandas import DatetimeIndex
 import pandas._testing as tm
 
 from pandas.tseries.holiday import (
@@ -264,3 +265,49 @@ def test_both_offset_observance_raises():
             offset=[DateOffset(weekday=SA(4))],
             observance=next_monday,
         )
+
+
+def test_half_open_interval_with_observance():
+    # Prompted by GH 49075
+    # Check for holidays that have a half-open date interval where
+    # they have either a start_date or end_date defined along
+    # with a defined observance pattern to make sure that the return type
+    # for Holiday.dates() remains consistent before & after the year that
+    # marks the 'edge' of the half-open date interval.
+
+    holiday_1 = Holiday(
+        "Arbitrary Holiday - start 2022-03-14",
+        start_date=datetime(2022, 3, 14),
+        month=3,
+        day=14,
+        observance=next_monday,
+    )
+    holiday_2 = Holiday(
+        "Arbitrary Holiday 2 - end 2022-03-20",
+        end_date=datetime(2022, 3, 20),
+        month=3,
+        day=20,
+        observance=next_monday,
+    )
+
+    class TestHolidayCalendar(AbstractHolidayCalendar):
+        rules = [
+            USMartinLutherKingJr,
+            holiday_1,
+            holiday_2,
+            USLaborDay,
+        ]
+
+    start = Timestamp("2022-08-01")
+    end = Timestamp("2022-08-31")
+    year_offset = DateOffset(years=5)
+    expected_results = DatetimeIndex([], dtype="datetime64[ns]", freq=None)
+    test_cal = TestHolidayCalendar()
+
+    date_interval_low = test_cal.holidays(start - year_offset, end - year_offset)
+    date_window_edge = test_cal.holidays(start, end)
+    date_interval_high = test_cal.holidays(start + year_offset, end + year_offset)
+
+    tm.assert_index_equal(date_interval_low, expected_results)
+    tm.assert_index_equal(date_window_edge, expected_results)
+    tm.assert_index_equal(date_interval_high, expected_results)

pandas/tseries/holiday.py

@@ -335,6 +335,9 @@ def _apply_rule(self, dates):
         -------
         Dates with rules applied
         """
+        if dates.empty:
+            return DatetimeIndex([])
+
         if self.observance is not None:
             return dates.map(lambda d: self.observance(d))
 
@@ -553,8 +556,7 @@ def merge(self, other, inplace: bool = False):
 class USFederalHolidayCalendar(AbstractHolidayCalendar):
     """
     US Federal Government Holiday Calendar based on rules specified by:
-    https://www.opm.gov/policy-data-oversight/
-       snow-dismissal-procedures/federal-holidays/
+    https://www.opm.gov/policy-data-oversight/pay-leave/federal-holidays/
     """
 
     rules = [