Implement support for structured and struct zarr-extension defined dtypes (#3781)

* Implement official support for `structured` and `struct` dtypes according to new extension.

* Update docs and add changelog

* Implement `struct` subclass instead of modifying `structured`

* Remove Structured from AnyDType to fix test_match_dtype_unique

* Revert removal of linting ignore flag

* Fix dtype support

* Fix import sort order -- Linting

* Resolve #3781 (comment) Only register `Struct` and allow it to appropriately pick up `structured` dtype

* Linting

---------

Co-authored-by: Davis Vann Bennett <davis.v.bennett@gmail.com>

Author: BrianMichell

changes/3781.feature.md

@@ -0,0 +1 @@
+Added `Struct` class (subclass of `Structured`) implementing the zarr-extensions `struct` dtype spec. Uses object-style field format and dict fill values. Legacy `Structured` remains available for backward compatibility.

docs/user-guide/data_types.md

@@ -229,6 +229,37 @@ here, it's possible to create it yourself: see [Adding New Data Types](#adding-n
 #### Struct-like
 - [Structured][zarr.dtype.Structured]
 
+!!! note "Zarr V3 Structured Data Types"
+
+    In Zarr V3, structured data types are specified using the `struct` extension defined in the
+    [zarr-extensions repository](https://github.com/zarr-developers/zarr-extensions/tree/main/data-types/struct).
+    The JSON representation uses an object format for fields:
+
+    ```json
+    {
+        "name": "struct",
+        "configuration": {
+            "fields": [
+                {"name": "x", "data_type": "float32"},
+                {"name": "y", "data_type": "int64"}
+            ]
+        }
+    }
+    ```
+
+    For backward compatibility, Zarr Python also accepts the legacy `structured` name with
+    tuple-format fields when reading existing data.
+
+    Fill values for structured types are represented as JSON objects mapping field names to values:
+
+    ```json
+    {"x": 1.5, "y": 42}
+    ```
+
+    When using structured types with multi-byte fields, the `bytes` codec must specify an
+    explicit `endian` parameter. If omitted, Zarr Python assumes little-endian for legacy
+    compatibility but emits a warning.
+
 ### Example Usage
 
 This section will demonstrates the basic usage of Zarr data types.

src/zarr/codecs/bytes.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import sys
+import warnings
 from dataclasses import dataclass, replace
 from enum import Enum
 from typing import TYPE_CHECKING
@@ -9,6 +10,7 @@
 from zarr.core.buffer import Buffer, NDBuffer
 from zarr.core.common import JSON, parse_enum, parse_named_configuration
 from zarr.core.dtype.common import HasEndianness
+from zarr.core.dtype.npy.structured import Struct
 
 if TYPE_CHECKING:
     from typing import Self
@@ -56,7 +58,20 @@ def to_dict(self) -> dict[str, JSON]:
             return {"name": "bytes", "configuration": {"endian": self.endian.value}}
 
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> Self:
-        if not isinstance(array_spec.dtype, HasEndianness):
+        if isinstance(array_spec.dtype, Struct):
+            if array_spec.dtype.has_multi_byte_fields():
+                if self.endian is None:
+                    warnings.warn(
+                        "Missing 'endian' for structured dtype with multi-byte fields. "
+                        "Assuming little-endian for legacy compatibility.",
+                        UserWarning,
+                        stacklevel=2,
+                    )
+                    return replace(self, endian=Endian.little)
+            else:
+                if self.endian is not None:
+                    return replace(self, endian=None)
+        elif not isinstance(array_spec.dtype, HasEndianness):
             if self.endian is not None:
                 return replace(self, endian=None)
         elif self.endian is None:

src/zarr/core/array.py

@@ -69,6 +69,7 @@
 )
 from zarr.core.config import config as zarr_config
 from zarr.core.dtype import (
+    Structured,
     VariableLengthBytes,
     VariableLengthUTF8,
     ZDType,
@@ -4879,10 +4880,13 @@ def default_serializer_v3(dtype: ZDType[Any, Any]) -> ArrayBytesCodec:
     length strings and variable length bytes have hard-coded serializers -- ``VLenUTF8Codec`` and
     ``VLenBytesCodec``, respectively.
 
+    Structured data types with multi-byte fields use ``BytesCodec`` with little-endian encoding.
     """
     serializer: ArrayBytesCodec = BytesCodec(endian=None)
 
-    if isinstance(dtype, HasEndianness):
+    if isinstance(dtype, HasEndianness) or (
+        isinstance(dtype, Structured) and dtype.has_multi_byte_fields()
+    ):
         serializer = BytesCodec(endian="little")
     elif isinstance(dtype, HasObjectCodec):
         if dtype.object_codec_id == "vlen-bytes":

src/zarr/core/dtype/__init__.py

@@ -21,7 +21,13 @@
 from zarr.core.dtype.npy.complex import Complex64, Complex128
 from zarr.core.dtype.npy.float import Float16, Float32, Float64
 from zarr.core.dtype.npy.int import Int8, Int16, Int32, Int64, UInt8, UInt16, UInt32, UInt64
-from zarr.core.dtype.npy.structured import Structured, StructuredJSON_V2, StructuredJSON_V3
+from zarr.core.dtype.npy.structured import (
+    Struct,
+    StructJSON_V3,
+    Structured,
+    StructuredJSON_V2,
+    StructuredJSON_V3,
+)
 from zarr.core.dtype.npy.time import (
     DateTime64,
     DateTime64JSON_V2,
@@ -75,6 +81,8 @@
     "RawBytes",
     "RawBytesJSON_V2",
     "RawBytesJSON_V3",
+    "Struct",
+    "StructJSON_V3",
     "Structured",
     "StructuredJSON_V2",
     "StructuredJSON_V3",
@@ -124,7 +132,7 @@
     | ComplexFloatDType
     | StringDType
     | BytesDType
-    | Structured
+    | Struct
     | TimeDType
     | VariableLengthBytes
 )
@@ -137,7 +145,7 @@
     *COMPLEX_FLOAT_DTYPE,
     *STRING_DTYPE,
     *BYTES_DTYPE,
-    Structured,
+    Struct,
     *TIME_DTYPE,
     VariableLengthBytes,
 )