Set ZArray default fill_value as NaT for datetime64

docs/releases.rst

@@ -23,6 +23,9 @@ Bug fixes
 
 - Exclude empty chunks during `ChunkDict` construction. (:pull:`198`)
   By `Gustavo Hidalgo <https://github.com/ghidalgo3>`_.
+- Fixed regression in `fill_value` handling for datetime dtypes making virtual
+  Zarr stores unreadable (:pr:`206`)
+  By `Timothy Hodson <https://github.com/thodson-usgs>`_
 
 Documentation
 ~~~~~~~~~~~~~

virtualizarr/tests/test_integration.py

@@ -4,6 +4,9 @@
 import xarray.testing as xrt
 
 from virtualizarr import open_virtual_dataset
+from virtualizarr.manifests.array import ManifestArray
+from virtualizarr.manifests.manifest import ChunkManifest
+from virtualizarr.zarr import ZArray
 
 
 @pytest.mark.parametrize(
@@ -166,6 +169,50 @@ def test_non_dimension_coordinates(self, tmpdir, format):
         # assert equal to original dataset
         xrt.assert_identical(roundtrip, ds)
 
+    def test_datetime64_dtype_fill_value(self, tmpdir, format):
+        chunks_dict = {
+            "0.0.0": {"path": "foo.nc", "offset": 100, "length": 100},
+        }
+        manifest = ChunkManifest(entries=chunks_dict)
+        chunks = (1, 1, 1)
+        shape = (1, 1, 1)
+        zarray = ZArray(
+            chunks=chunks,
+            compressor={"id": "zlib", "level": 1},
+            dtype=np.dtype("<M8[ns]"),
+            # fill_value=0.0,
+            filters=None,
+            order="C",
+            shape=shape,
+            zarr_format=2,
+        )
+        marr1 = ManifestArray(zarray=zarray, chunkmanifest=manifest)
+        ds = xr.Dataset(
+            {
+                "a": xr.DataArray(
+                    marr1,
+                    attrs={
+                        "_FillValue": np.datetime64("1970-01-01T00:00:00.000000000")
+                    },
+                )
+            }
+        )
+
+        if format == "dict":
+            # write those references to an in-memory kerchunk-formatted references dictionary
+            ds_refs = ds.virtualize.to_kerchunk(format=format)
+
+            # use fsspec to read the dataset from the kerchunk references dict
+            roundtrip = xr.open_dataset(ds_refs, engine="kerchunk")
+        else:
+            # write those references to disk as kerchunk references format
+            ds.virtualize.to_kerchunk(f"{tmpdir}/refs.{format}", format=format)
+
+            # use fsspec to read the dataset from disk via the kerchunk references
+            roundtrip = xr.open_dataset(f"{tmpdir}/refs.{format}", engine="kerchunk")
+
+        assert roundtrip.a.attrs == ds.a.attrs
+
 
 def test_open_scalar_variable(tmpdir):
     # regression test for GH issue #100

virtualizarr/zarr.py

@@ -32,13 +32,14 @@
 )  # just the .zattrs (for one array or for the whole store/group)
 FillValueT = bool | str | float | int | list | None
 
-ZARR_DEFAULT_FILL_VALUE: dict[np.dtype, FillValueT] = {
+ZARR_DEFAULT_FILL_VALUE: dict[str, FillValueT] = {
     # numpy dtypes's hierarchy lets us avoid checking for all the widths
     # https://numpy.org/doc/stable/reference/arrays.scalars.html
-    np.dtype("bool"): False,
-    np.dtype("int"): 0,
-    np.dtype("float"): 0.0,
-    np.dtype("complex"): [0.0, 0.0],
+    np.dtype("bool").kind: False,
+    np.dtype("int").kind: 0,
+    np.dtype("float").kind: 0.0,
+    np.dtype("complex").kind: [0.0, 0.0],
+    np.dtype("datetime64").kind: 0,
 }
 """
 The value and format of the fill_value depend on the `data_type` of the array.
@@ -67,7 +68,7 @@ class ZArray(BaseModel):
     chunks: tuple[int, ...]
     compressor: dict | None = None
     dtype: np.dtype
-    fill_value: FillValueT = Field(default=0.0, validate_default=True)
+    fill_value: FillValueT = Field(None, validate_default=True)
     filters: list[dict] | None = None
     order: Literal["C", "F"]
     shape: tuple[int, ...]
@@ -90,7 +91,7 @@ def __post_init__(self) -> None:
     @model_validator(mode="after")
     def _check_fill_value(self) -> Self:
         if self.fill_value is None:
-            self.fill_value = ZARR_DEFAULT_FILL_VALUE.get(self.dtype, 0.0)
+            self.fill_value = ZARR_DEFAULT_FILL_VALUE.get(self.dtype.kind, 0.0)
         return self
 
     @property