Set longdouble=False in cftime.date2num within the date encoding context (#7171)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: spencerkclark

doc/whats-new.rst

@@ -59,6 +59,10 @@ Internal Changes
   By `Maximilian Roos <https://github.com/max-sixty>`_.
 
 
+- Explicitly specify ``longdouble=False`` in :py:func:`cftime.date2num` when
+  encoding times to preserve existing behavior and prevent future errors when it
+  is eventually set to ``True`` by default in cftime (:pull:`7171`).  By
+  `Spencer Clark <https://github.com/spencerkclark>`_.
 
 .. _whats-new.2022.10.0:
 

xarray/coding/times.py

@@ -581,7 +581,18 @@ def _encode_datetime_with_cftime(dates, units, calendar):
         dates = dates.astype("M8[us]").astype(datetime)
 
     def encode_datetime(d):
-        return np.nan if d is None else cftime.date2num(d, units, calendar)
+        # Since netCDF files do not support storing float128 values, we ensure
+        # that float64 values are used by setting longdouble=False in num2date.
+        # This try except logic can be removed when xarray's minimum version of
+        # cftime is at least 1.6.2.
+        try:
+            return (
+                np.nan
+                if d is None
+                else cftime.date2num(d, units, calendar, longdouble=False)
+            )
+        except TypeError:
+            return np.nan if d is None else cftime.date2num(d, units, calendar)
 
     return np.array([encode_datetime(d) for d in dates.ravel()]).reshape(dates.shape)
 

xarray/tests/test_coding_times.py

@@ -1080,7 +1080,14 @@ def test__encode_datetime_with_cftime() -> None:
     times = cftime.num2date([0, 1], "hours since 2000-01-01", calendar)
 
     encoding_units = "days since 2000-01-01"
-    expected = cftime.date2num(times, encoding_units, calendar)
+    # Since netCDF files do not support storing float128 values, we ensure that
+    # float64 values are used by setting longdouble=False in num2date.  This try
+    # except logic can be removed when xarray's minimum version of cftime is at
+    # least 1.6.2.
+    try:
+        expected = cftime.date2num(times, encoding_units, calendar, longdouble=False)
+    except TypeError:
+        expected = cftime.date2num(times, encoding_units, calendar)
     result = _encode_datetime_with_cftime(times, encoding_units, calendar)
     np.testing.assert_equal(result, expected)