Fix fill_value=null zarr v2 to v3 conversion for string dtypes

docs/releases.md

@@ -8,6 +8,12 @@
   ([809](https://github.com/zarr-developers/VirtualiZarr/pull/809)).
   By [Julia Signell](https://github.com/jsignell).
 
+### Bug fixes
+
+- Fix setting `fill_value` for Zarr V2 arrays if data type is a subtype of integer or float.
+  ([#845](https://github.com/zarr-developers/VirtualiZarr/pull/845)).
+  By [Hauke Schulz](https://github.com/observingClouds).
+
 ## v2.2.1 (17th November 2025)
 
 ### Bug fixes

virtualizarr/parsers/zarr.py

@@ -4,10 +4,11 @@
 from abc import ABC, abstractmethod
 from collections.abc import Iterable
 from pathlib import Path
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, cast
 
 import zarr
 from zarr.api.asynchronous import open_group as open_group_async
+from zarr.core.dtype import parse_dtype
 from zarr.core.group import GroupMetadata
 from zarr.core.metadata import ArrayV3Metadata
 from zarr.storage import ObjectStore
@@ -194,20 +195,13 @@ def get_metadata(self, zarr_array: ZarrArrayType) -> ArrayV3Metadata:
 
         if v2_metadata.fill_value is None:
             v2_dict = v2_metadata.to_dict()
-            v2_dict["fill_value"] = 0
+            v2_dtype = parse_dtype(cast(Any, v2_dict["dtype"]), zarr_format=2)
+            fill_value = v2_dtype.default_scalar()
+            v2_dict["fill_value"] = (
+                fill_value.item() if hasattr(fill_value, "item") else fill_value
+            )
             temp_v2 = ArrayV2Metadata.from_dict(v2_dict)
             v3_metadata = _convert_array_metadata(temp_v2)
-
-            # Replace with proper default for the data type
-            default_scalar = v3_metadata.data_type.default_scalar()
-            fill_value = (
-                default_scalar.item()
-                if hasattr(default_scalar, "item")
-                else default_scalar
-            )
-            v3_dict = v3_metadata.to_dict()
-            v3_dict["fill_value"] = fill_value
-            v3_metadata = ArrayV3Metadata.from_dict(v3_dict)
         else:
             # Normal conversion; allow other errors to propagate.
             v3_metadata = _convert_array_metadata(v2_metadata)

virtualizarr/tests/test_parsers/test_zarr.py

@@ -264,7 +264,17 @@ async def get_meta():
 
 
 @SKIP_OLDER_ZARR_PYTHON
-def test_v2_metadata_with_none_fill_value():
+@pytest.mark.parametrize(
+    "dtype",
+    [
+        "int32",
+        "uint8",
+        "float64",
+        "bool",
+        "U10",
+    ],
+)
+def test_v2_metadata_with_none_fill_value(dtype):
     """Test V2 metadata conversion when fill_value is None."""
     import asyncio
 
@@ -273,7 +283,7 @@ def test_v2_metadata_with_none_fill_value():
     _ = zarr.create(
         shape=(5, 10),
         chunks=(5, 5),
-        dtype="int32",
+        dtype=dtype,
         store=store,
         zarr_format=2,
         fill_value=None,