separate CFMaskCoder test for multiple missing_values/__FillValues conflict, use _FillValue for missing_value if available

Author: kmuehlbauer

xarray/coding/variables.py

@@ -243,7 +243,8 @@ def encode(self, variable: Variable, name: T_Name = None):
                     data = duck_array_ops.fillna(data, fill_value)
 
             if mv is not None:
-                # Only use mv if _FillValue isn't available
+                # Use _FillValue if available to align missing_value to prevent issues
+                # when decoding
                 # Ensure missing_value is cast to same dtype as data's
                 encoding["missing_value"] = attrs.get("_FillValue", dtype.type(mv))
                 fill_value = pop_to(encoding, attrs, "missing_value", name=name)

xarray/tests/test_coding.py

@@ -51,9 +51,15 @@ def test_CFMaskCoder_encode_missing_fill_values_conflict(data, encoding) -> None
     assert encoded.dtype == encoded.attrs["missing_value"].dtype
     assert encoded.dtype == encoded.attrs["_FillValue"].dtype
 
+
+def test_CFMaskCoder_multiple_missing_values_conflict():
+    data = np.array([0.0, -1.0, 1.0])
+    attrs = dict(_FillValue=np.float64(1e20), missing_value=np.float64(1e21))
+    original = xr.Variable(("x",), data, attrs=attrs)
     with pytest.warns(variables.SerializationWarning):
-        roundtripped = decode_cf_variable("foo", encoded)
-        assert_identical(roundtripped, original)
+        decoded = decode_cf_variable("foo", original)
+    with pytest.raises(ValueError):
+        encode_cf_variable(decoded)
 
 
 def test_CFMaskCoder_missing_value() -> None: