fix: zero default scalar for bytes fields in structured dtypes

changes/4495.bugfix.md

@@ -0,0 +1 @@
+Structured data types now default byte-string (`S`) and raw-bytes (`V`) fields to zero bytes. Before, an array with a raw-bytes field failed to create without an explicit fill value, and byte-string fields defaulted to `b"0"`.

src/zarr/core/dtype/npy/structured.py

@@ -579,7 +579,7 @@ def default_scalar(self) -> np.void:
                 # zero count is representable by viewing the integer storage.
                 value = np.zeros(1, dtype=dtype.byteorder + "i8").view(dtype)[0]
             else:
-                value = np.array([0], dtype=dtype)[0]
+                value = np.zeros(1, dtype=dtype)[0]
             values.append(value)
         return self._cast_scalar_unchecked(tuple(values))
 

tests/test_dtype/test_npy/test_structured.py

@@ -392,3 +392,12 @@ def test_subarray_structured_dtype_raises() -> None:
     dtype = np.dtype([("f0", "i4", (2,))])
     with pytest.raises(ValueError, match="field 'f0' is a subarray"):
         Struct.from_native_dtype(dtype)
+
+
+@pytest.mark.parametrize("field_dtype", ["S4", "V4"])
+def test_default_scalar_bytes_field_is_zero(field_dtype: str) -> None:
+    """
+    The default scalar of a bytes field is zero bytes, as for the same dtype outside a struct.
+    """
+    dtype = np.dtype([("a", field_dtype), ("b", "<f4")])
+    assert Struct.from_native_dtype(dtype).default_scalar().tobytes() == bytes(dtype.itemsize)