Add time_unit argument to CFTimeIndex.to_datetimeindex

doc/whats-new.rst

@@ -56,6 +56,13 @@ New Features
   By `Sam Levang <https://github.com/slevang>`_.
 - Improve the error message raised when no key is matching the available variables in a dataset.  (:pull:`9943`)
   By `Jimmy Westling <https://github.com/illviljan>`_.
+- Added a ``time_unit`` argument to :py:meth:`CFTimeIndex.to_datetimeindex`.
+  Note that in a future version of xarray,
+  :py:meth:`CFTimeIndex.to_datetimeindex` will return a microsecond-resolution
+  :py:class:`pandas.DatetimeIndex` instead of a nanosecond-resolution
+  :py:class:`pandas.DatetimeIndex` (:pull:`9965`). By `Spencer Clark
+  <https://github.com/spencerkclark>`_ and `Kai Mühlbauer
+  <https://github.com/kmuehlbauer>`_.
 - :py:meth:`DatasetGroupBy.first` and :py:meth:`DatasetGroupBy.last` can now use ``flox`` if available. (:issue:`9647`)
   By `Deepak Cherian <https://github.com/dcherian>`_.
 

xarray/coding/cftimeindex.py

@@ -42,9 +42,8 @@
 from __future__ import annotations
 
 import math
-import warnings
 from datetime import timedelta
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, Optional
 
 import numpy as np
 import pandas as pd
@@ -58,7 +57,8 @@
 )
 from xarray.core.common import _contains_cftime_datetimes
 from xarray.core.options import OPTIONS
-from xarray.core.utils import attempt_import, is_scalar
+from xarray.core.types import PDDatetimeUnitOptions
+from xarray.core.utils import attempt_import, emit_user_level_warning, is_scalar
 
 if TYPE_CHECKING:
     from xarray.coding.cftime_offsets import BaseCFTimeOffset
@@ -239,6 +239,8 @@ class CFTimeIndex(pd.Index):
     cftime_range
     """
 
+    _data: np.ndarray
+
     year = _field_accessor("year", "The year of the datetime")
     month = _field_accessor("month", "The month of the datetime")
     day = _field_accessor("day", "The days of the datetime")
@@ -544,14 +546,18 @@ def __rsub__(self, other):
                 "that can be expressed at the nanosecond resolution."
             ) from err
 
-    def to_datetimeindex(self, unsafe=False):
+    def to_datetimeindex(
+        self, unsafe: bool = False, time_unit: Optional[PDDatetimeUnitOptions] = None
+    ) -> pd.DatetimeIndex:
         """If possible, convert this index to a pandas.DatetimeIndex.
 
         Parameters
         ----------
         unsafe : bool
-            Flag to turn off warning when converting from a CFTimeIndex with
-            a non-standard calendar to a DatetimeIndex (default ``False``).
+            Flag to turn off calendar mismatch warnings (default ``False``).
+        time_unit : str
+            Time resolution of resulting DatetimeIndex. Can be one of `"s"`,
+            ``"ms"``, ``"us"``, or ``"ns"`` (default ``"ns"``).
 
         Returns
         -------
@@ -561,45 +567,68 @@ def to_datetimeindex(self, unsafe=False):
         ------
         ValueError
             If the CFTimeIndex contains dates that are not possible in the
-            standard calendar or outside the nanosecond-precision range.
+            standard calendar or outside the range representable by the
+            specified ``time_unit``.
 
         Warns
         -----
         RuntimeWarning
-            If converting from a non-standard calendar to a DatetimeIndex.
+            If converting from a non-standard calendar, or a Gregorian
+            calendar with dates prior to the reform (1582-10-15).
 
         Warnings
         --------
-        Note that for non-standard calendars, this will change the calendar
-        type of the index.  In that case the result of this method should be
-        used with caution.
+        Note that for non-proleptic Gregorian calendars, this will change the
+        calendar type of the index. In that case the result of this method
+        should be used with caution.
 
         Examples
         --------
         >>> times = xr.cftime_range("2000", periods=2, calendar="gregorian")
         >>> times
         CFTimeIndex([2000-01-01 00:00:00, 2000-01-02 00:00:00],
                     dtype='object', length=2, calendar='standard', freq=None)
-        >>> times.to_datetimeindex()
-        DatetimeIndex(['2000-01-01', '2000-01-02'], dtype='datetime64[us]', freq=None)
+        >>> times.to_datetimeindex(time_unit="ns")
+        DatetimeIndex(['2000-01-01', '2000-01-02'], dtype='datetime64[ns]', freq=None)
         """
 
         if not self._data.size:
             return pd.DatetimeIndex([])
 
-        # transform to us-resolution is needed for DatetimeIndex
-        nptimes = cftime_to_nptime(self, time_unit="us")
+        if time_unit is None:
+            emit_user_level_warning(
+                "In a future version of xarray to_datetimeindex will default "
+                "to returning a 'us'-resolution DatetimeIndex instead of a "
+                "'ns'-resolution DatetimeIndex. This warning can be silenced "
+                "by explicitly passing the `time_unit` keyword argument.",
+                FutureWarning,
+            )
+            time_unit = "ns"
+
+        nptimes = cftime_to_nptime(self, time_unit=time_unit)
         calendar = infer_calendar_name(self)
         if calendar not in _STANDARD_CALENDARS and not unsafe:
-            warnings.warn(
+            emit_user_level_warning(
                 "Converting a CFTimeIndex with dates from a non-standard "
-                f"calendar, {calendar!r}, to a pandas.DatetimeIndex, which uses dates "
-                "from the standard calendar.  This may lead to subtle errors "
-                "in operations that depend on the length of time between "
-                "dates.",
+                f"calendar, {calendar!r}, to a pandas.DatetimeIndex, which "
+                "uses dates from the standard calendar.  This may lead to "
+                "subtle errors in operations that depend on the length of "
+                "time between dates.",
                 RuntimeWarning,
-                stacklevel=2,
             )
+        if calendar == "standard" and not unsafe:
+            reform_date = self.date_type(1582, 10, 15)
+            if self.min() < reform_date:
+                emit_user_level_warning(
+                    "Converting a CFTimeIndex with dates from a Gregorian "
+                    "calendar that fall before the reform date of 1582-10-15 "
+                    "to a pandas.DatetimeIndex. During this time period the "
+                    "Gregorian calendar and the proleptic Gregorian calendar "
+                    "of the DatetimeIndex do not exactly align. This warning "
+                    "can be silenced by setting unsafe=True.",
+                    RuntimeWarning,
+                )
+
         return pd.DatetimeIndex(nptimes)
 
     def strftime(self, date_format):

xarray/tests/test_accessor_dt.py

@@ -519,7 +519,9 @@ def test_cftime_strftime_access(data) -> None:
     date_format = "%Y%m%d%H"
     result = data.time.dt.strftime(date_format)
     datetime_array = xr.DataArray(
-        xr.coding.cftimeindex.CFTimeIndex(data.time.values).to_datetimeindex(),
+        xr.coding.cftimeindex.CFTimeIndex(data.time.values).to_datetimeindex(
+            time_unit="ns"
+        ),
         name="stftime",
         coords=data.time.coords,
         dims=data.time.dims,

xarray/tests/test_cftime_offsets.py

@@ -1670,9 +1670,9 @@ def test_new_to_legacy_freq_pd_freq_passthrough(freq, expected):
         pytest.param("-1YE", marks=requires_pandas_3),
     ),
 )
-def test_cftime_range_same_as_pandas(start, end, freq):
+def test_cftime_range_same_as_pandas(start, end, freq) -> None:
     result = date_range(start, end, freq=freq, calendar="standard", use_cftime=True)
-    result = result.to_datetimeindex()
+    result = result.to_datetimeindex(time_unit="ns")
     expected = date_range(start, end, freq=freq, use_cftime=False)
 
     np.testing.assert_array_equal(result, expected)
@@ -1694,8 +1694,8 @@ def test_cftime_range_no_freq(start, end, periods):
     when freq is not provided, but start, end and periods are.
     """
     # Generate date ranges using cftime_range
-    result = cftime_range(start=start, end=end, periods=periods)
-    result = result.to_datetimeindex()
+    cftimeindex = cftime_range(start=start, end=end, periods=periods)
+    result = cftimeindex.to_datetimeindex(time_unit="ns")
     expected = pd.date_range(start=start, end=end, periods=periods)
 
     np.testing.assert_array_equal(result, expected)

xarray/tests/test_cftimeindex.py

@@ -1219,43 +1219,59 @@ def test_strftime_of_cftime_array(calendar):
 @requires_cftime
 @pytest.mark.parametrize("calendar", _ALL_CALENDARS)
 @pytest.mark.parametrize("unsafe", [False, True])
-def test_to_datetimeindex(calendar, unsafe):
+def test_to_datetimeindex(calendar, unsafe) -> None:
     index = xr.cftime_range("2000", periods=5, calendar=calendar)
-    expected = pd.date_range("2000", periods=5, unit="us")
+    expected = pd.date_range("2000", periods=5, unit="ns")
 
     if calendar in _NON_STANDARD_CALENDARS and not unsafe:
         with pytest.warns(RuntimeWarning, match="non-standard"):
-            result = index.to_datetimeindex()
+            result = index.to_datetimeindex(time_unit="ns")
     else:
-        result = index.to_datetimeindex(unsafe=unsafe)
+        result = index.to_datetimeindex(unsafe=unsafe, time_unit="ns")
 
     assert result.equals(expected)
     np.testing.assert_array_equal(result, expected)
     assert isinstance(result, pd.DatetimeIndex)
 
 
+@requires_cftime
+def test_to_datetimeindex_future_warning() -> None:
+    index = xr.cftime_range("2000", periods=5)
+    expected = pd.date_range("2000", periods=5, unit="ns")
+    with pytest.warns(FutureWarning, match="In a future version"):
+        result = index.to_datetimeindex()
+    assert result.equals(expected)
+    assert result.dtype == expected.dtype
+
+
 @requires_cftime
 @pytest.mark.parametrize("calendar", _ALL_CALENDARS)
-def test_to_datetimeindex_out_of_range(calendar):
+def test_to_datetimeindex_out_of_range(calendar) -> None:
     index = xr.cftime_range("0001", periods=5, calendar=calendar)
-    # todo: suggestion from code review:
-    #  - still warn when converting from a non-standard calendar
-    #  to a proleptic Gregorian calendar
-    #  - also warn when converting from a Gregorian calendar
-    #  to a proleptic Gregorian calendar when dates fall before the reform
-    if calendar in _NON_STANDARD_CALENDARS:
-        with pytest.warns(RuntimeWarning, match="non-standard"):
-            index.to_datetimeindex()
+    with pytest.raises(ValueError, match="0001"):
+        index.to_datetimeindex(time_unit="ns")
+
+
+@requires_cftime
+@pytest.mark.parametrize("unsafe", [False, True])
+def test_to_datetimeindex_gregorian_pre_reform(unsafe) -> None:
+    index = xr.cftime_range("1582", periods=5, calendar="gregorian")
+    if unsafe:
+        result = index.to_datetimeindex(time_unit="us", unsafe=unsafe)
     else:
-        index.to_datetimeindex()
+        with pytest.warns(RuntimeWarning, match="reform"):
+            result = index.to_datetimeindex(time_unit="us", unsafe=unsafe)
+    expected = pd.date_range("1582", periods=5, unit="us")
+    assert result.equals(expected)
+    assert result.dtype == expected.dtype
 
 
 @requires_cftime
 @pytest.mark.parametrize("calendar", ["all_leap", "360_day"])
-def test_to_datetimeindex_feb_29(calendar):
+def test_to_datetimeindex_feb_29(calendar) -> None:
     index = xr.cftime_range("2001-02-28", periods=2, calendar=calendar)
     with pytest.raises(ValueError, match="29"):
-        index.to_datetimeindex()
+        index.to_datetimeindex(time_unit="ns")
 
 
 @pytest.mark.xfail(reason="fails on pandas main branch")
@@ -1269,12 +1285,12 @@ def test_multiindex():
 @requires_cftime
 @pytest.mark.parametrize("freq", ["3663s", "33min", "2h"])
 @pytest.mark.parametrize("method", ["floor", "ceil", "round"])
-def test_rounding_methods_against_datetimeindex(freq, method):
+def test_rounding_methods_against_datetimeindex(freq, method) -> None:
     # for now unit="us" seems good enough
-    expected = pd.date_range("2000-01-02T01:03:51", periods=10, freq="1777s", unit="us")
+    expected = pd.date_range("2000-01-02T01:03:51", periods=10, freq="1777s", unit="ns")
     expected = getattr(expected, method)(freq)
     result = xr.cftime_range("2000-01-02T01:03:51", periods=10, freq="1777s")
-    result = getattr(result, method)(freq).to_datetimeindex()
+    result = getattr(result, method)(freq).to_datetimeindex(time_unit="ns")
     assert result.equals(expected)
 
 

xarray/tests/test_cftimeindex_resample.py

@@ -97,7 +97,9 @@ def compare_against_pandas(
         ).mean()
     # TODO (benbovy - flexible indexes): update when CFTimeIndex is a xarray Index subclass
     result_cftimeindex["time"] = (
-        result_cftimeindex.xindexes["time"].to_pandas_index().to_datetimeindex()
+        result_cftimeindex.xindexes["time"]
+        .to_pandas_index()
+        .to_datetimeindex(time_unit="ns")
     )
     xr.testing.assert_identical(result_cftimeindex, result_datetimeindex)
 
@@ -181,7 +183,7 @@ def test_calendars(calendar: str) -> None:
     # TODO (benbovy - flexible indexes): update when CFTimeIndex is a xarray Index subclass
     new_pd_index = da_cftime.xindexes["time"].to_pandas_index()
     assert isinstance(new_pd_index, CFTimeIndex)  # shouldn't that be a pd.Index?
-    da_cftime["time"] = new_pd_index.to_datetimeindex()
+    da_cftime["time"] = new_pd_index.to_datetimeindex(time_unit="ns")
     xr.testing.assert_identical(da_cftime, da_datetime)
 
 

xarray/tests/test_groupby.py

@@ -1870,7 +1870,7 @@ def test_resample(
         def resample_as_pandas(array, *args, **kwargs):
             array_ = array.copy(deep=True)
             if use_cftime:
-                array_["time"] = times.to_datetimeindex()
+                array_["time"] = times.to_datetimeindex(time_unit="ns")
             result = DataArray.from_series(
                 array_.to_series().resample(*args, **kwargs).mean()
             )
@@ -2323,7 +2323,7 @@ def test_resample(
         def resample_as_pandas(ds, *args, **kwargs):
             ds_ = ds.copy(deep=True)
             if use_cftime:
-                ds_["time"] = times.to_datetimeindex()
+                ds_["time"] = times.to_datetimeindex(time_unit="ns")
             result = Dataset.from_dataframe(
                 ds_.to_dataframe().resample(*args, **kwargs).mean()
             )

xarray/tests/test_missing.py

@@ -606,10 +606,12 @@ def test_get_clean_interp_index_cf_calendar(cf_da, calendar):
 @requires_cftime
 @pytest.mark.parametrize("calendar", ["gregorian", "proleptic_gregorian"])
 @pytest.mark.parametrize("freq", ["1D", "1ME", "1YE"])
-def test_get_clean_interp_index_dt(cf_da, calendar, freq):
+def test_get_clean_interp_index_dt(cf_da, calendar, freq) -> None:
     """In the gregorian case, the index should be proportional to normal datetimes."""
     g = cf_da(calendar, freq=freq)
-    g["stime"] = xr.Variable(data=g.time.to_index().to_datetimeindex(), dims=("time",))
+    g["stime"] = xr.Variable(
+        data=g.time.to_index().to_datetimeindex(time_unit="ns"), dims=("time",)
+    )
 
     gi = get_clean_interp_index(g, "time")
     si = get_clean_interp_index(g, "time", use_coordinate="stime")