fixed datetime serialization

Author: TomAugspurger

src/zarr/core/metadata.py

@@ -256,13 +256,21 @@ def _json_convert(o: Any) -> Any:
             if isinstance(o, np.dtype):
                 return str(o)
             if np.isscalar(o):
-                # convert numpy scalar to python type, and pass
-                # python types through
-                out = getattr(o, "item", lambda: o)()
-                if isinstance(out, complex):
-                    # python complex types are not JSON serializable, so we use the
-                    # serialization defined in the zarr v3 spec
-                    return [out.real, out.imag]
+                out: Any
+                if hasattr(o, "dtype") and o.dtype.kind == "M" and hasattr(o, "view"):
+                    # https://github.com/zarr-developers/zarr-python/issues/2119
+                    # `.item()` on a datetime type might or might not return an
+                    # integer, depending on the value.
+                    # Explicitly cast to an int first, and then grab .item()
+                    out = o.view("i8").item()
+                else:
+                    # convert numpy scalar to python type, and pass
+                    # python types through
+                    out = getattr(o, "item", lambda: o)()
+                    if isinstance(out, complex):
+                        # python complex types are not JSON serializable, so we use the
+                        # serialization defined in the zarr v3 spec
+                        return [out.real, out.imag]
                 return out
             if isinstance(o, Enum):
                 return o.name

src/zarr/testing/strategies.py

@@ -1,3 +1,4 @@
+import re
 from typing import Any
 
 import hypothesis.extra.numpy as npst
@@ -101,7 +102,14 @@ def arrays(
     root = Group.create(store)
     fill_value_args: tuple[Any, ...] = tuple()
     if nparray.dtype.kind == "M":
-        fill_value_args = ("ns",)
+        m = re.search("\[(.+)\]", nparray.dtype.str)
+        if not m:
+            raise ValueError(f"Couldn't find precision for dtype '{nparray.dtype}.")
+
+        fill_value_args = (
+            # e.g. ns, D
+            m.groups()[0],
+        )
 
     a = root.create_array(
         array_path,

tests/v3/test_metadata/test_v3.py

@@ -1,10 +1,12 @@
 from __future__ import annotations
 
+import json
 import re
 from typing import TYPE_CHECKING, Literal
 
 from zarr.abc.codec import Codec
 from zarr.codecs.bytes import BytesCodec
+from zarr.core.buffer import default_buffer_prototype
 from zarr.core.chunk_key_encodings import DefaultChunkKeyEncoding, V2ChunkKeyEncoding
 
 if TYPE_CHECKING:
@@ -230,3 +232,24 @@ def test_metadata_to_dict(
         observed.pop("chunk_key_encoding")
         expected.pop("chunk_key_encoding")
     assert observed == expected
+
+
+@pytest.mark.parametrize("fill_value", [-1, 0, 1, 2932897])
+@pytest.mark.parametrize("precision", ["ns", "D"])
+async def test_datetime_metadata(fill_value: int, precision: str):
+    metadata_dict = {
+        "zarr_format": 3,
+        "node_type": "array",
+        "shape": (1,),
+        "chunk_grid": {"name": "regular", "configuration": {"chunk_shape": (1,)}},
+        "data_type": f"<M8[{precision}]",
+        "chunk_key_encoding": {"name": "default", "separator": "."},
+        "codecs": (),
+        "fill_value": np.datetime64(fill_value, precision),
+    }
+    metadata = ArrayV3Metadata.from_dict(metadata_dict)
+    # ensure there isn't a TypeError here.
+    d = metadata.to_buffer_dict(default_buffer_prototype())
+
+    result = json.loads(d["zarr.json"].to_bytes())
+    assert result["fill_value"] == fill_value