refactor: define codec and data type classes upstream in a subpackage

packages/zarr-interfaces/README.md

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+# zarr-interfaces
+
+Interface definitions (ABCs and protocols) for zarr codecs and data types.
+
+This package provides the abstract base classes and protocols that external
+codec and data type implementations should subclass or implement. It has
+minimal dependencies (only numpy) and does not depend on zarr-python itself.
+
+## Usage
+
+```python
+from zarr_interfaces.codec.v1 import ArrayArrayCodec, ArrayBytesCodec, BytesBytesCodec
+from zarr_interfaces.data_type.v1 import ZDType
+```
+
+Interfaces are versioned under a `v1` namespace to support future evolution
+without breaking existing implementations.

packages/zarr-interfaces/pyproject.toml

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+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[project]
+name = "zarr-interfaces"
+version = "0.1.0"
+description = "Interface definitions (ABCs and protocols) for zarr codecs and data types"
+readme = "README.md"
+license = "BSD-3-Clause"
+requires-python = ">=3.12"
+dependencies = [
+    "numpy>=2",
+]
+
+[project.urls]
+homepage = "https://github.com/zarr-developers/zarr-python"

packages/zarr-interfaces/src/zarr_interfaces/codec/v1.py

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+"""Codec interface definitions (v1).
+
+This module defines the abstract interfaces for zarr codecs.
+External codec implementations should subclass ``ArrayArrayCodec``,
+``ArrayBytesCodec``, or ``BytesBytesCodec`` from this module.
+
+The ``Buffer`` and ``NDBuffer`` types here are protocols — they define
+the structural interface that zarr's concrete buffer types implement.
+Codec authors should type against these protocols, not zarr's concrete
+buffer classes.
+"""
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, ClassVar, Protocol, Self, runtime_checkable
+
+if TYPE_CHECKING:
+    import numpy as np
+    import numpy.typing as npt
+
+    from zarr_interfaces.data_type.v1 import JSON, TBaseDType, TBaseScalar, ZDType
+
+
+# ---------------------------------------------------------------------------
+# Buffer protocols
+# ---------------------------------------------------------------------------
+
+
+class Buffer(Protocol):
+    """Protocol for a flat contiguous memory block (bytes-like)."""
+
+    def __len__(self) -> int: ...
+    def __getitem__(self, key: slice) -> Buffer: ...
+
+
+class NDBuffer(Protocol):
+    """Protocol for an N-dimensional array buffer."""
+
+    @property
+    def dtype(self) -> np.dtype[np.generic]: ...
+
+    @property
+    def shape(self) -> tuple[int, ...]: ...
+
+    def as_ndarray_like(self) -> npt.NDArray[np.generic]: ...
+
+    @classmethod
+    def from_ndarray_like(cls, data: npt.NDArray[np.generic]) -> NDBuffer: ...
+
+    def transpose(self, axes: tuple[int, ...]) -> NDBuffer: ...
+
+    def __getitem__(self, key: object) -> NDBuffer: ...
+
+    def __setitem__(self, key: object, value: object) -> None: ...
+
+
+# ---------------------------------------------------------------------------
+# ArraySpec protocol
+# ---------------------------------------------------------------------------
+
+
+class ArraySpec(Protocol):
+    """Protocol for the specification of a chunk's metadata."""
+
+    @property
+    def shape(self) -> tuple[int, ...]: ...
+
+    @property
+    def dtype(self) -> ZDType[TBaseDType, TBaseScalar]: ...
+
+    @property
+    def fill_value(self) -> object: ...
+
+    @property
+    def ndim(self) -> int: ...
+
+
+# ---------------------------------------------------------------------------
+# Codec input/output type aliases
+# ---------------------------------------------------------------------------
+
+type CodecInput = NDBuffer | Buffer
+type CodecOutput = NDBuffer | Buffer
+
+
+# ---------------------------------------------------------------------------
+# Sync codec protocol
+# ---------------------------------------------------------------------------
+
+
+@runtime_checkable
+class SupportsSyncCodec[CI: CodecInput, CO: CodecOutput](Protocol):
+    """Protocol for codecs that support synchronous encode/decode.
+
+    The type parameters mirror ``BaseCodec``: ``CI`` is the decoded type
+    and ``CO`` is the encoded type.
+    """
+
+    def _decode_sync(self, chunk_data: CO, chunk_spec: ArraySpec) -> CI: ...
+
+    def _encode_sync(self, chunk_data: CI, chunk_spec: ArraySpec) -> CO | None: ...
+
+
+# ---------------------------------------------------------------------------
+# Codec ABCs
+# ---------------------------------------------------------------------------
+
+
+@dataclass(frozen=True)
+class BaseCodec[CI: CodecInput, CO: CodecOutput](ABC):
+    """Generic base class for codecs.
+
+    Subclass ``ArrayArrayCodec``, ``ArrayBytesCodec``, or
+    ``BytesBytesCodec`` instead of this class directly.
+    """
+
+    is_fixed_size: ClassVar[bool]
+
+    @classmethod
+    def from_dict(cls, data: dict[str, JSON]) -> Self:
+        """Create an instance from a JSON dictionary."""
+        return cls(**data)  # type: ignore[arg-type]
+
+    def to_dict(self) -> dict[str, JSON]:
+        """Serialize this codec to a JSON dictionary."""
+        raise NotImplementedError
+
+    @abstractmethod
+    def compute_encoded_size(self, input_byte_length: int, chunk_spec: ArraySpec) -> int:
+        """Return the encoded byte length for a given input byte length."""
+        ...
+
+    def resolve_metadata(self, chunk_spec: ArraySpec) -> ArraySpec:
+        """Return the chunk spec after encoding by this codec.
+
+        Override this for codecs that change shape, dtype, or fill value.
+        """
+        return chunk_spec
+
+    def evolve_from_array_spec(self, array_spec: ArraySpec) -> Self:
+        """Fill in codec parameters that can be inferred from array metadata."""
+        return self
+
+    def validate(
+        self,
+        *,
+        shape: tuple[int, ...],
+        dtype: ZDType[TBaseDType, TBaseScalar],
+        chunk_grid: object,
+    ) -> None:
+        """Validate that this codec is compatible with the array metadata.
+
+        The default implementation does nothing. Override to add checks.
+        """
+
+    async def _decode_single(self, chunk_data: CO, chunk_spec: ArraySpec) -> CI:
+        """Decode a single chunk. Override this or ``_decode_sync``."""
+        raise NotImplementedError
+
+    async def decode(
+        self,
+        chunks_and_specs: Iterable[tuple[CO | None, ArraySpec]],
+    ) -> Iterable[CI | None]:
+        """Decode a batch of chunks."""
+        results: list[CI | None] = []
+        for chunk_data, chunk_spec in chunks_and_specs:
+            if chunk_data is not None:
+                results.append(await self._decode_single(chunk_data, chunk_spec))
+            else:
+                results.append(None)
+        return results
+
+    async def _encode_single(self, chunk_data: CI, chunk_spec: ArraySpec) -> CO | None:
+        """Encode a single chunk. Override this or ``_encode_sync``."""
+        raise NotImplementedError
+
+    async def encode(
+        self,
+        chunks_and_specs: Iterable[tuple[CI | None, ArraySpec]],
+    ) -> Iterable[CO | None]:
+        """Encode a batch of chunks."""
+        results: list[CO | None] = []
+        for chunk_data, chunk_spec in chunks_and_specs:
+            if chunk_data is not None:
+                results.append(await self._encode_single(chunk_data, chunk_spec))
+            else:
+                results.append(None)
+        return results
+
+
+class ArrayArrayCodec(BaseCodec[NDBuffer, NDBuffer]):
+    """Base class for array-to-array codecs (e.g. transpose, scale_offset)."""
+
+
+class ArrayBytesCodec(BaseCodec[NDBuffer, Buffer]):
+    """Base class for array-to-bytes codecs (e.g. bytes, sharding)."""
+
+
+class BytesBytesCodec(BaseCodec[Buffer, Buffer]):
+    """Base class for bytes-to-bytes codecs (e.g. gzip, zstd)."""

packages/zarr-interfaces/src/zarr_interfaces/data_type/v1.py

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+"""Data type interface definitions (v1).
+
+This module defines the abstract interface for zarr data types.
+External data type implementations should subclass ``ZDType`` from this
+module. The interface is intentionally minimal and stable.
+"""
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+from typing import (
+    ClassVar,
+    Literal,
+    Self,
+    TypeGuard,
+    overload,
+)
+
+import numpy as np
+
+# JSON-like type for serialization
+type JSON = str | int | float | bool | dict[str, JSON] | list[JSON] | None
+type ZarrFormat = Literal[2, 3]
+
+# Bounds for the scalar and dtype type parameters
+type TBaseScalar = np.generic | str | bytes
+type TBaseDType = np.dtype[np.generic]
+
+# JSON representations of data types
+type DTypeJSON = JSON
+type DTypeSpec_V2 = str | list[tuple[str, DTypeJSON]]
+type DTypeSpec_V3 = str | dict[str, JSON]
+
+
+@dataclass(frozen=True, kw_only=True, slots=True)
+class ZDType[DType: TBaseDType, Scalar: TBaseScalar](ABC):
+    """Abstract base class for wrapping native array data types.
+
+    Subclasses must implement all abstract methods to support serialization,
+    deserialization, and scalar handling for their native data type.
+
+    Type Parameters
+    ---------------
+    DType
+        The native data type (e.g. ``np.dtype[np.float64]``).
+    Scalar
+        The scalar type produced by this data type (e.g. ``np.float64``).
+    """
+
+    dtype_cls: ClassVar[type[TBaseDType]]
+    _zarr_v3_name: ClassVar[str]
+
+    @classmethod
+    def _check_native_dtype(cls: type[Self], dtype: TBaseDType) -> TypeGuard[DType]:
+        """Check that a native data type matches ``dtype_cls``."""
+        return type(dtype) is cls.dtype_cls
+
+    @classmethod
+    @abstractmethod
+    def from_native_dtype(cls: type[Self], dtype: TBaseDType) -> Self:
+        """Create an instance from a native data type."""
+        ...
+
+    @abstractmethod
+    def to_native_dtype(self: Self) -> DType:
+        """Return the native data type wrapped by this instance."""
+        ...
+
+    @classmethod
+    @abstractmethod
+    def _from_json_v2(cls: type[Self], data: DTypeJSON) -> Self: ...
+
+    @classmethod
+    @abstractmethod
+    def _from_json_v3(cls: type[Self], data: DTypeJSON) -> Self: ...
+
+    @classmethod
+    def from_json(cls: type[Self], data: DTypeJSON, *, zarr_format: ZarrFormat) -> Self:
+        """Create an instance from JSON metadata."""
+        if zarr_format == 2:
+            return cls._from_json_v2(data)
+        if zarr_format == 3:
+            return cls._from_json_v3(data)
+        raise ValueError(f"zarr_format must be 2 or 3, got {zarr_format}")
+
+    @overload
+    def to_json(self, zarr_format: Literal[2]) -> DTypeSpec_V2: ...
+
+    @overload
+    def to_json(self, zarr_format: Literal[3]) -> DTypeSpec_V3: ...
+
+    @abstractmethod
+    def to_json(self, zarr_format: ZarrFormat) -> DTypeSpec_V2 | DTypeSpec_V3:
+        """Serialize this data type to JSON."""
+        ...
+
+    @abstractmethod
+    def _check_scalar(self, data: object) -> bool:
+        """Check that a python object is a valid scalar for this data type."""
+        ...
+
+    @abstractmethod
+    def cast_scalar(self, data: object) -> Scalar:
+        """Cast a python object to the scalar type of this data type."""
+        ...
+
+    @abstractmethod
+    def default_scalar(self) -> Scalar:
+        """Return the default scalar value for this data type."""
+        ...
+
+    @abstractmethod
+    def from_json_scalar(self: Self, data: JSON, *, zarr_format: ZarrFormat) -> Scalar:
+        """Deserialize a JSON value to a scalar."""
+        ...
+
+    @abstractmethod
+    def to_json_scalar(self, data: object, *, zarr_format: ZarrFormat) -> JSON:
+        """Serialize a scalar value to JSON."""
+        ...

packages/zarr-interfaces/src/zarr_interfaces/metadata/v1.py

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+"""Metadata protocol (v1).
+
+Defines the structural interface for objects that can be serialized
+to and deserialized from JSON dictionaries.
+"""
+
+from __future__ import annotations
+
+from typing import Protocol, Self, runtime_checkable
+
+type JSON = str | int | float | bool | dict[str, JSON] | list[JSON] | None
+
+
+@runtime_checkable
+class Metadata(Protocol):
+    """Protocol for objects that round-trip through JSON dictionaries."""
+
+    @classmethod
+    def from_dict(cls, data: dict[str, JSON]) -> Self:
+        """Create an instance from a JSON dictionary."""
+        ...
+
+    def to_dict(self) -> dict[str, JSON]:
+        """Serialize to a JSON dictionary."""
+        ...