Restore ability to specify _FillValue as Python native integers

Author: djhoese

xarray/coding/variables.py

@@ -520,9 +520,19 @@ def encode(self, variable: Variable, name: T_Name = None) -> Variable:
             # trying to get it from encoding, resort to an int with the same precision as data.dtype if not available
             signed_dtype = np.dtype(encoding.get("dtype", f"i{data.dtype.itemsize}"))
             if "_FillValue" in attrs:
-                new_fill = np.array(attrs["_FillValue"])
-                # use view here to prevent OverflowError
-                attrs["_FillValue"] = new_fill.view(signed_dtype).item()
+                try:
+                    # user provided the on-disk signed fill
+                    new_fill = signed_dtype.type(attrs["_FillValue"])
+                except OverflowError:
+                    # user provided the in-memory unsigned fill, convert to signed type
+                    unsigned_dtype = np.dtype(f"u{signed_dtype.itemsize}")
+                    # use view here to prevent OverflowError
+                    new_fill = (
+                        np.array(attrs["_FillValue"], dtype=unsigned_dtype)
+                        .view(signed_dtype)
+                        .item()
+                    )
+                attrs["_FillValue"] = new_fill
             data = duck_array_ops.astype(duck_array_ops.around(data), signed_dtype)
 
             return Variable(dims, data, attrs, encoding, fastpath=True)

xarray/tests/test_backends.py

@@ -166,7 +166,7 @@ def create_encoded_masked_and_scaled_data(dtype: np.dtype) -> Dataset:
 
 def create_unsigned_masked_scaled_data(dtype: np.dtype) -> Dataset:
     encoding = {
-        "_FillValue": np.int8(-1),
+        "_FillValue": 255,
         "_Unsigned": "true",
         "dtype": "i1",
         "add_offset": dtype.type(10),
@@ -925,7 +925,7 @@ def test_roundtrip_mask_and_scale(self, decoded_fn, encoded_fn, dtype) -> None:
                 assert decoded.variables[k].dtype == actual.variables[k].dtype
             assert_allclose(decoded, actual, decode_bytes=False)
 
-    @pytest.mark.parametrize("fillvalue", [np.int8(-1), np.uint8(255)])
+    @pytest.mark.parametrize("fillvalue", [np.int8(-1), np.uint8(255), -1, 255])
     def test_roundtrip_unsigned(self, fillvalue):
         # regression/numpy2 test for
         encoding = {