Adapt to upstream change in resampling with a Day frequency

doc/whats-new.rst

@@ -104,7 +104,14 @@ Bug Fixes
   entries; ``to_dataframe`` indexes by the union of stored entries across all
   sparse variables sharing the same dims (:issue:`4007`).
   By `patnr <https://github.com/patnr>`_.
+- Following `pandas-dev/pandas#64793`_, ensure that resampling an array to a
+  ``Day`` frequency along a :py:class:`xarray.CFTimeIndex` produces the same
+  results as resampling to an equivalent ``Hour`` frequency, including with the
+  use of ``origin`` and ``offset`` options (:pull:`11546`). This effectively
+  rolls back the resample-related changes introduced in :pull:`10650`. By
+  `Spencer Clark <https://github.com/spencerkclark>`_.
 
+.. _`pandas-dev/pandas#64793`: https://github.com/pandas-dev/pandas/pull/64793
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/resample_cftime.py

@@ -45,8 +45,9 @@
 import pandas as pd
 
 from xarray.coding.cftime_offsets import (
-    CFTIME_TICKS,
     BaseCFTimeOffset,
+    Day,
+    Hour,
     MonthEnd,
     QuarterEnd,
     Tick,
@@ -86,19 +87,19 @@ def __init__(
         self.freq = to_offset(freq)
         self.origin = origin
 
-        if not isinstance(self.freq, CFTIME_TICKS):
+        if not isinstance(self.freq, (Tick, Day)):
             if offset is not None:
                 message = (
                     "The 'offset' keyword does not take effect when "
-                    "resampling with a 'freq' that is not Tick-like (h, m, s, "
-                    "ms, us)"
+                    "resampling with a 'freq' that is not Tick-like (D, h, m, "
+                    "s, ms, us)"
                 )
                 emit_user_level_warning(message, category=RuntimeWarning)
             if origin != "start_day":
                 message = (
                     "The 'origin' keyword does not take effect when "
-                    "resampling with a 'freq' that is not Tick-like (h, m, s, "
-                    "ms, us)"
+                    "resampling with a 'freq' that is not Tick-like (D, h, m, "
+                    "s, ms, us)"
                 )
                 emit_user_level_warning(message, category=RuntimeWarning)
 
@@ -338,7 +339,9 @@ def _get_range_edges(
     last : cftime.datetime
         Corrected ending datetime object for resampled CFTimeIndex range.
     """
-    if isinstance(freq, Tick):
+    if isinstance(freq, (Tick, Day)):
+        if isinstance(freq, Day):
+            freq = Hour(24 * freq.n)
         first, last = _adjust_dates_anchored(
             first, last, freq, closed=closed, origin=origin, offset=offset
         )

xarray/tests/__init__.py

@@ -198,7 +198,7 @@ def get_dask_chunkmanager():
 has_flox, requires_flox = _importorskip("flox")
 has_netcdf, requires_netcdf = _importorskip("netcdf")
 has_pandas_3, requires_pandas_3 = _importorskip("pandas", "3.0.0")
-
+has_pandas_3_1, requires_pandas_3_1 = _importorskip("pandas", "3.1.0.dev0")
 
 # some special cases
 has_scipy_or_netCDF4 = has_scipy or has_netCDF4

xarray/tests/test_cftimeindex_resample.py

@@ -9,14 +9,14 @@
 
 import xarray as xr
 from xarray.coding.cftime_offsets import (
-    CFTIME_TICKS,
     Day,
+    Tick,
     to_offset,
 )
 from xarray.coding.cftimeindex import CFTimeIndex
 from xarray.core.resample_cftime import CFTimeGrouper
 from xarray.core.types import PDDatetimeUnitOptions
-from xarray.tests import has_pandas_3
+from xarray.tests import has_pandas_3_1
 
 cftime = pytest.importorskip("cftime")
 
@@ -60,9 +60,11 @@
 
 
 def has_tick_resample_freq(freqs):
-    resample_freq, _ = freqs
+    _, resample_freq = freqs
     resample_freq_as_offset = to_offset(resample_freq)
-    return isinstance(resample_freq_as_offset, CFTIME_TICKS)
+    # As of pandas 3.1, in the context of resampling a timezone-naive index,
+    # Day is treated like a Tick frequency (pandas-dev/pandas#64793).
+    return isinstance(resample_freq_as_offset, (Tick, Day))
 
 
 def has_non_tick_resample_freq(freqs):
@@ -137,6 +139,9 @@ def da(index) -> xr.DataArray:
 @pytest.mark.parametrize("offset", [None, "5s"], ids=lambda x: f"{x}")
 def test_resample_with_tick_resample_freq(freqs, closed, label, offset) -> None:
     initial_freq, resample_freq = freqs
+    resample_freq_as_offset = to_offset(resample_freq)
+    if isinstance(resample_freq_as_offset, Day) and not has_pandas_3_1:
+        pytest.skip("Only valid for pandas >= 3.1")
     start = "2000-01-01T12:07:01"
     origin = "start"
 
@@ -165,9 +170,6 @@ def test_resample_with_tick_resample_freq(freqs, closed, label, offset) -> None:
 @pytest.mark.parametrize("label", [None, "left", "right"])
 def test_resample_with_non_tick_resample_freq(freqs, closed, label) -> None:
     initial_freq, resample_freq = freqs
-    resample_freq_as_offset = to_offset(resample_freq)
-    if isinstance(resample_freq_as_offset, Day) and not has_pandas_3:
-        pytest.skip("Only valid for pandas >= 3.0")
     start = "2000-01-01T12:07:01"
 
     # Set offset and origin to their default values since they have no effect