Merge branch 'main' into keep-attrs

Author: max-sixty

doc/whats-new.rst

@@ -115,6 +115,9 @@ Bug fixes
 
 - Fix the ``align_chunks`` parameter on the :py:meth:`~xarray.Dataset.to_zarr` method, it was not being
   passed to the underlying :py:meth:`~xarray.backends.api` method (:issue:`10501`, :pull:`10516`).
+- Fix error when encoding an empty :py:class:`numpy.datetime64` array
+  (:issue:`10722`, :pull:`10723`). By `Spencer Clark
+  <https://github.com/spencerkclark>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/coding/times.py

@@ -1065,9 +1065,12 @@ def _eagerly_encode_cf_datetime(
             # parse with cftime instead
             raise OutOfBoundsDatetime
         assert np.issubdtype(dates.dtype, "datetime64")
-        if calendar in ["standard", "gregorian"] and np.nanmin(dates).astype(
-            "=M8[us]"
-        ).astype(datetime) < datetime(1582, 10, 15):
+        if (
+            calendar in ["standard", "gregorian"]
+            and dates.size > 0
+            and np.nanmin(dates).astype("=M8[us]").astype(datetime)
+            < datetime(1582, 10, 15)
+        ):
             raise_gregorian_proleptic_gregorian_mismatch_error = True
 
         time_unit, ref_date = _unpack_time_unit_and_ref_date(units)

xarray/tests/test_coding_times.py

@@ -2198,3 +2198,24 @@ def test_roundtrip_0size_timedelta(time_unit: PDDatetimeUnitOptions) -> None:
         decoded.load()
     assert decoded.dtype == np.dtype("=m8[s]")
     assert decoded.encoding == encoding
+
+
+def test_roundtrip_empty_datetime64_array(time_unit: PDDatetimeUnitOptions) -> None:
+    # Regression test for GitHub issue #10722.
+    encoding = {
+        "units": "days since 1990-1-1",
+        "dtype": np.dtype("float64"),
+        "calendar": "standard",
+    }
+    times = date_range("2000", periods=0, unit=time_unit)
+    variable = Variable(["time"], times, encoding=encoding)
+
+    encoded = conventions.encode_cf_variable(variable, name="foo")
+    assert encoded.dtype == np.dtype("float64")
+
+    decode_times = CFDatetimeCoder(time_unit=time_unit)
+    roundtripped = conventions.decode_cf_variable(
+        "foo", encoded, decode_times=decode_times
+    )
+    assert_identical(variable, roundtripped)
+    assert roundtripped.dtype == variable.dtype