Special case str dtype in array creation

src/zarr/codecs/_v2.py

@@ -36,7 +36,7 @@ async def _decode_single(
             chunk_numpy_array = ensure_ndarray(chunk_bytes.as_array_like())
 
         # ensure correct dtype
-        if str(chunk_numpy_array.dtype) != chunk_spec.dtype:
+        if str(chunk_numpy_array.dtype) != chunk_spec.dtype and not chunk_spec.dtype.hasobject:
             chunk_numpy_array = chunk_numpy_array.view(chunk_spec.dtype)
 
         return get_ndbuffer_class().from_numpy_array(chunk_numpy_array)

src/zarr/core/array.py

@@ -35,6 +35,7 @@
     ShapeLike,
     ZarrFormat,
     concurrent_map,
+    parse_dtype,
     parse_shapelike,
     product,
 )
@@ -365,16 +366,17 @@ async def create(
     ) -> AsyncArray[ArrayV2Metadata] | AsyncArray[ArrayV3Metadata]:
         store_path = await make_store_path(store)
 
+        dtype_parsed = parse_dtype(dtype, zarr_format)
         shape = parse_shapelike(shape)
 
         if chunks is not None and chunk_shape is not None:
             raise ValueError("Only one of chunk_shape or chunks can be provided.")
 
-        dtype = np.dtype(dtype)
         if chunks:
-            _chunks = normalize_chunks(chunks, shape, dtype.itemsize)
+            _chunks = normalize_chunks(chunks, shape, dtype_parsed.itemsize)
         else:
-            _chunks = normalize_chunks(chunk_shape, shape, dtype.itemsize)
+            _chunks = normalize_chunks(chunk_shape, shape, dtype_parsed.itemsize)
+
         result: AsyncArray[ArrayV3Metadata] | AsyncArray[ArrayV2Metadata]
         if zarr_format == 3:
             if dimension_separator is not None:
@@ -396,7 +398,7 @@ async def create(
             result = await cls._create_v3(
                 store_path,
                 shape=shape,
-                dtype=dtype,
+                dtype=dtype_parsed,
                 chunk_shape=_chunks,
                 fill_value=fill_value,
                 chunk_key_encoding=chunk_key_encoding,
@@ -406,6 +408,14 @@ async def create(
                 exists_ok=exists_ok,
             )
         elif zarr_format == 2:
+            if dtype is str or dtype == "str":
+                # another special case: zarr v2 added the vlen-utf8 codec
+                vlen_codec: dict[str, JSON] = {"id": "vlen-utf8"}
+                if filters and not any(x["id"] == "vlen-utf8" for x in filters):
+                    filters = list(filters) + [vlen_codec]
+                else:
+                    filters = [vlen_codec]
+
             if codecs is not None:
                 raise ValueError(
                     "codecs cannot be used for arrays with version 2. Use filters and compressor instead."
@@ -419,7 +429,7 @@ async def create(
             result = await cls._create_v2(
                 store_path,
                 shape=shape,
-                dtype=dtype,
+                dtype=dtype_parsed,
                 chunks=_chunks,
                 dimension_separator=dimension_separator,
                 fill_value=fill_value,

src/zarr/core/common.py

@@ -14,6 +14,10 @@
     overload,
 )
 
+import numpy as np
+
+from zarr.core.strings import _STRING_DTYPE
+
 if TYPE_CHECKING:
     from collections.abc import Awaitable, Callable, Iterator
 
@@ -151,3 +155,13 @@ def parse_order(data: Any) -> Literal["C", "F"]:
     if data in ("C", "F"):
         return cast(Literal["C", "F"], data)
     raise ValueError(f"Expected one of ('C', 'F'), got {data} instead.")
+
+
+def parse_dtype(dtype: Any, zarr_format: ZarrFormat) -> np.dtype[Any]:
+    if dtype is str or dtype == "str":
+        if zarr_format == 2:
+            # special case as object
+            return np.dtype("object")
+        else:
+            return _STRING_DTYPE
+    return np.dtype(dtype)

src/zarr/core/metadata/v2.py

@@ -321,7 +321,7 @@ def _default_fill_value(dtype: np.dtype[Any]) -> Any:
     """
     if dtype.kind == "S":
         return b""
-    elif dtype.kind == "U":
+    elif dtype.kind in "UO":
         return ""
     else:
         return dtype.type(0)

tests/v3/test_codecs/test_vlen.py

@@ -11,7 +11,7 @@
 from zarr.core.strings import _NUMPY_SUPPORTS_VLEN_STRING
 from zarr.storage.common import StorePath
 
-numpy_str_dtypes: list[type | str | None] = [None, str, np.dtypes.StrDType]
+numpy_str_dtypes: list[type | str | None] = [None, str, "str", np.dtypes.StrDType]
 expected_zarr_string_dtype: np.dtype[Any]
 if _NUMPY_SUPPORTS_VLEN_STRING:
     numpy_str_dtypes.append(np.dtypes.StringDType)

tests/v3/test_v2.py

@@ -2,6 +2,7 @@
 from collections.abc import Iterator
 from typing import Any
 
+import numcodecs.vlen
 import numpy as np
 import pytest
 from numcodecs import Delta
@@ -44,7 +45,7 @@ def test_simple(store: StorePath) -> None:
         ("float64", 0.0),
         ("|S1", b""),
         ("|U1", ""),
-        ("object", 0),
+        ("object", ""),
         (str, ""),
     ],
 )
@@ -53,7 +54,12 @@ def test_implicit_fill_value(store: StorePath, dtype: str, fill_value: Any) -> N
     assert arr.metadata.fill_value is None
     assert arr.metadata.to_dict()["fill_value"] is None
     result = arr[:]
-    expected = np.full(arr.shape, fill_value, dtype=dtype)
+    if dtype is str:
+        # special case
+        numpy_dtype = np.dtype(object)
+    else:
+        numpy_dtype = np.dtype(dtype)
+    expected = np.full(arr.shape, fill_value, dtype=numpy_dtype)
     np.testing.assert_array_equal(result, expected)
 
 
@@ -106,3 +112,14 @@ async def test_v2_encode_decode(dtype):
     data = zarr.open_array(store=store, path="foo")[:]
     expected = np.full((3,), b"X", dtype=dtype)
     np.testing.assert_equal(data, expected)
+
+
+@pytest.mark.parametrize("dtype", [str, "str"])
+async def test_create_dtype_str(dtype: Any) -> None:
+    arr = zarr.create(shape=3, dtype=dtype, zarr_format=2)
+    assert arr.dtype.kind == "O"
+    assert arr.metadata.to_dict()["dtype"] == "|O"
+    assert arr.metadata.filters == (numcodecs.vlen.VLenUTF8(),)
+    arr[:] = ["a", "bb", "ccc"]
+    result = arr[:]
+    np.testing.assert_array_equal(result, np.array(["a", "bb", "ccc"], dtype="object"))