Port negative frequency fix for pandas.date_range to cftime_range (#8999)

Author: spencerkclark

doc/whats-new.rst

@@ -60,6 +60,12 @@ Breaking changes
 
 Bug fixes
 ~~~~~~~~~
+- Following `an upstream bug fix
+  <https://github.com/pandas-dev/pandas/issues/56147>`_ to
+  :py:func:`pandas.date_range`, date ranges produced by
+  :py:func:`xarray.cftime_range` with negative frequencies will now fall fully
+  within the bounds of the provided start and end dates (:pull:`8999`). By
+  `Spencer Clark <https://github.com/spencerkclark>`_.
 
 
 Internal Changes

xarray/coding/cftime_offsets.py

@@ -845,7 +845,12 @@ def _generate_range(start, end, periods, offset):
     A generator object
     """
     if start:
-        start = offset.rollforward(start)
+        # From pandas GH 56147 / 56832 to account for negative direction and
+        # range bounds
+        if offset.n >= 0:
+            start = offset.rollforward(start)
+        else:
+            start = offset.rollback(start)
 
     if periods is None and end < start and offset.n >= 0:
         end = None

xarray/tests/__init__.py

@@ -130,6 +130,7 @@ def _importorskip(
 has_numexpr, requires_numexpr = _importorskip("numexpr")
 has_flox, requires_flox = _importorskip("flox")
 has_pandas_ge_2_2, __ = _importorskip("pandas", "2.2")
+has_pandas_3, requires_pandas_3 = _importorskip("pandas", "3.0.0.dev0")
 
 
 # some special cases

xarray/tests/test_cftime_offsets.py

@@ -42,6 +42,7 @@
     has_cftime,
     has_pandas_ge_2_2,
     requires_cftime,
+    requires_pandas_3,
 )
 
 cftime = pytest.importorskip("cftime")
@@ -1354,7 +1355,7 @@ def test_calendar_specific_month_end_negative_freq(
 ) -> None:
     year = 2000  # Use a leap-year to highlight calendar differences
     result = cftime_range(
-        start="2000-12",
+        start="2001",
         end="2000",
         freq="-2ME",
         calendar=calendar,
@@ -1464,7 +1465,7 @@ def test_date_range_errors() -> None:
         ("2020-02-01", "QE-DEC", "noleap", "gregorian", True, "2020-03-31", True),
         ("2020-02-01", "YS-FEB", "noleap", "gregorian", True, "2020-02-01", True),
         ("2020-02-01", "YE-FEB", "noleap", "gregorian", True, "2020-02-29", True),
-        ("2020-02-01", "-1YE-FEB", "noleap", "gregorian", True, "2020-02-29", True),
+        ("2020-02-01", "-1YE-FEB", "noleap", "gregorian", True, "2019-02-28", True),
         ("2020-02-28", "3h", "all_leap", "gregorian", False, "2020-02-28", True),
         ("2020-03-30", "ME", "360_day", "gregorian", False, "2020-03-31", True),
         ("2020-03-31", "ME", "gregorian", "360_day", None, "2020-03-30", False),
@@ -1724,7 +1725,17 @@ def test_new_to_legacy_freq_pd_freq_passthrough(freq, expected):
 @pytest.mark.parametrize("start", ("2000", "2001"))
 @pytest.mark.parametrize("end", ("2000", "2001"))
 @pytest.mark.parametrize(
-    "freq", ("MS", "-1MS", "YS", "-1YS", "ME", "-1ME", "YE", "-1YE")
+    "freq",
+    (
+        "MS",
+        pytest.param("-1MS", marks=requires_pandas_3),
+        "YS",
+        pytest.param("-1YS", marks=requires_pandas_3),
+        "ME",
+        pytest.param("-1ME", marks=requires_pandas_3),
+        "YE",
+        pytest.param("-1YE", marks=requires_pandas_3),
+    ),
 )
 def test_cftime_range_same_as_pandas(start, end, freq):
     result = date_range(start, end, freq=freq, calendar="standard", use_cftime=True)