maxrjones · Oct 17, 2026 · Oct 17, 2026
diff --git a/changes/4522.removal.md b/changes/4522.removal.md
@@ -0,0 +1 @@
+The Zarr-Python 2 keyword arguments `synchronizer`, `chunk_store`, `cache_metadata`, `cache_attrs`, `object_codec`, `read_only` and `meta_array` accepted by `zarr.create`, `zarr.group` and `zarr.open_group` (and the functions that wrap them, such as `zarr.zeros`) have never had any effect in Zarr-Python 3. Passing one of them now emits a `ZarrDeprecationWarning` instead of a "not yet implemented" `ZarrRuntimeWarning`. They will be removed in a release after April 2027. Calls that do not pass these arguments are unaffected.
diff --git a/docs/user-guide/v3_migration.md b/docs/user-guide/v3_migration.md
@@ -231,9 +231,9 @@ The following functions / methods have not been ported to Zarr-Python 3:
 - `zarr.copy_all` ([issue #2407](https://github.com/zarr-developers/zarr-python/issues/2407))
 - `zarr.copy_store` ([issue #2407](https://github.com/zarr-developers/zarr-python/issues/2407))
 
-The following features (corresponding to function arguments to functions in
-  `zarr`) have not been ported to Zarr-Python 3. Using these features
-  will raise a warning or a `NotImplementedError`:
+The following function arguments from Zarr-Python 2 are accepted but ignored by
+  Zarr-Python 3. Passing them emits a `ZarrDeprecationWarning`, and they will be
+  removed in a release after April 2027:
 
 - `cache_attrs`
 - `cache_metadata`

diff --git a/src/zarr/api/asynchronous.py b/src/zarr/api/asynchronous.py
@@ -44,7 +44,6 @@
     GroupNotFoundError,
     NodeTypeValidationError,
     ZarrDeprecationWarning,
-    ZarrRuntimeWarning,
     ZarrUserWarning,
 )
 from zarr.storage import StorePath
@@ -109,17 +108,23 @@ def _infer_overwrite(mode: AccessModeLiteral) -> bool:
     return mode in _OVERWRITE_MODES
 
 
-def _warn_unimplemented_kwargs(kwargs: dict[str, Any]) -> None:
+def _warn_ignored_kwargs(kwargs: dict[str, Any]) -> None:
     """
-    Emit a "not yet implemented" warning for each provided keyword argument that is not None.
+    Emit a deprecation warning for each provided keyword argument that is not None.
+
+    These are Zarr-Python 2 keyword arguments that have no effect in Zarr-Python 3.
 
     `kwargs` maps a keyword argument name to its supplied value. The `stacklevel` is chosen
     so the warning points at the caller of the public API function (the same location as an
     inline `warnings.warn(..., stacklevel=2)` would).
     """
     for name, value in kwargs.items():
         if value is not None:
-            warnings.warn(f"{name} is not yet implemented", ZarrRuntimeWarning, stacklevel=3)
+            warnings.warn(
+                f"{name} is ignored, and will be removed in a release after April 2027",
+                ZarrDeprecationWarning,
+                stacklevel=3,
+            )
 
 
 def _get_shape_chunks(a: ArrayLike | Any) -> tuple[tuple[int, ...] | None, tuple[int, ...] | None]:
@@ -717,18 +722,15 @@ async def group(
         If True, delete any pre-existing data in `store` at `path` before
         creating the group.
     chunk_store : StoreLike or None, default=None
-        Separate storage for chunks. Not implemented.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     cache_attrs : bool, optional
-        If True (default), user attributes will be cached for attribute read
-        operations. If False, user attributes are reloaded from the store prior
-        to all attribute read operations.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     synchronizer : object, optional
-        Array synchronizer.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     path : str, optional
         Group path within store.
     meta_array : array-like, optional
-        An array instance to use for determining arrays to create and return
-        to users. Use `numpy.empty(())` by default.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     zarr_format : {2, 3, None}, optional
         The zarr format to use when saving.
     storage_options : dict
@@ -840,23 +842,18 @@ async def open_group(
         (fail if exists). On a store that cannot delete keys, 'w' raises an
         error instead of replacing an existing node.
     cache_attrs : bool, optional
-        If True (default), user attributes will be cached for attribute read
-        operations. If False, user attributes are reloaded from the store prior
-        to all attribute read operations.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     synchronizer : object, optional
-        Array synchronizer.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     path : str, optional
         Group path within store.
     chunk_store : StoreLike or None, default=None
-        Separate storage for chunks. See the
-        [storage documentation in the user guide][user-guide-store-like]
-        for a description of all valid StoreLike values.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     storage_options : dict
         If using an fsspec URL to create the store, these will be passed to
         the backend implementation. Ignored otherwise.
     meta_array : array-like, optional
-        An array instance to use for determining arrays to create and return
-        to users. Use `numpy.empty(())` by default.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     attributes : dict
         A dictionary of JSON-serializable values with user-defined attributes.
     use_consolidated : bool or str, default None
@@ -882,7 +879,7 @@ async def open_group(
         The new group.
     """
 
-    _warn_unimplemented_kwargs(
+    _warn_ignored_kwargs(
         {
             "cache_attrs": cache_attrs,
             "synchronizer": synchronizer,
@@ -986,15 +983,14 @@ async def create(
         [storage documentation in the user guide][user-guide-store-like]
         for a description of all valid StoreLike values.
     synchronizer : object, optional
-        Array synchronizer.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     overwrite : bool, optional
         If True, delete all pre-existing data in `store` at `path` before
         creating the array.
     path : str, optional
         Path under which array is stored.
     chunk_store : StoreLike or None, default=None
-        Separate storage for chunks. If not provided, `store` will be used
-        for storage of both chunks and metadata.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     filters : Iterable[Codec] | Literal["auto"], optional
         Iterable of filters to apply to each chunk of the array, in order, before serializing that
         chunk to bytes.
@@ -1015,18 +1011,13 @@ async def create(
 
         To create an array with no filters, provide an empty iterable or the value `None`.
     cache_metadata : bool, optional
-        If True, array configuration metadata will be cached for the
-        lifetime of the object. If False, array metadata will be reloaded
-        prior to all data access and modification operations (may incur
-        overhead depending on storage and data access pattern).
+        Ignored. Deprecated and will be removed in a release after April 2027.
     cache_attrs : bool, optional
-        If True (default), user attributes will be cached for attribute read
-        operations. If False, user attributes are reloaded from the store prior
-        to all attribute read operations.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     read_only : bool, optional
-        True if array should be protected against modification.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     object_codec : Codec, optional
-        A codec to encode object arrays, only needed if dtype=object.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     dimension_separator : {'.', '/'}, optional
         Separator placed between the dimensions of a chunk.
         Zarr format 2 only. Zarr format 3 arrays should use `chunk_key_encoding` instead.
@@ -1043,7 +1034,7 @@ async def create(
         which instructs Zarr to choose the default Zarr format value defined in the
         runtime configuration.
     meta_array : array-like, optional
-        Not implemented.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     attributes : dict[str, JSON], optional
         A dictionary of user attributes to store with the array.
     chunk_shape : ChunksLike, optional
@@ -1101,7 +1092,7 @@ async def create(
     if zarr_format is None:
         zarr_format = _default_zarr_format()
 
-    _warn_unimplemented_kwargs(
+    _warn_ignored_kwargs(
         {
             "synchronizer": synchronizer,
             "chunk_store": chunk_store,

diff --git a/src/zarr/api/synchronous.py b/src/zarr/api/synchronous.py
@@ -452,18 +452,15 @@ def group(
         If True, delete any pre-existing data in `store` at `path` before
         creating the group.
     chunk_store : StoreLike or None, default=None
-        Separate storage for chunks. Not implemented.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     cache_attrs : bool, optional
-        If True (default), user attributes will be cached for attribute read
-        operations. If False, user attributes are reloaded from the store prior
-        to all attribute read operations.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     synchronizer : object, optional
-        Array synchronizer.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     path : str, optional
         Group path within store.
     meta_array : array-like, optional
-        An array instance to use for determining arrays to create and return
-        to users. Use `numpy.empty(())` by default.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     zarr_format : {2, 3, None}, optional
         The zarr format to use when saving.
     storage_options : dict
@@ -522,23 +519,18 @@ def open_group(
         (fail if exists). On a store that cannot delete keys, 'w' raises an
         error instead of replacing an existing node.
     cache_attrs : bool, optional
-        If True (default), user attributes will be cached for attribute read
-        operations. If False, user attributes are reloaded from the store prior
-        to all attribute read operations.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     synchronizer : object, optional
-        Array synchronizer.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     path : str, optional
         Group path within store.
     chunk_store : StoreLike or None, default=None
-        Separate storage for chunks. See the
-        [storage documentation in the user guide][user-guide-store-like]
-        for a description of all valid StoreLike values.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     storage_options : dict
         If using an fsspec URL to create the store, these will be passed to
         the backend implementation. Ignored otherwise.
     meta_array : array-like, optional
-        An array instance to use for determining arrays to create and return
-        to users. Use `numpy.empty(())` by default.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     attributes : dict
         A dictionary of JSON-serializable values with user-defined attributes.
     use_consolidated : bool or str, default None
@@ -703,15 +695,14 @@ def create(
         [storage documentation in the user guide][user-guide-store-like]
         for a description of all valid StoreLike values.
     synchronizer : object, optional
-        Array synchronizer.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     overwrite : bool, optional
         If True, delete all pre-existing data in `store` at `path` before
         creating the array.
     path : str, optional
         Path under which array is stored.
     chunk_store : StoreLike or None, default=None
-        Separate storage for chunks. If not provided, `store` will be used
-        for storage of both chunks and metadata.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     filters : Iterable[Codec] | Literal["auto"], optional
         Iterable of filters to apply to each chunk of the array, in order, before serializing that
         chunk to bytes.
@@ -732,18 +723,13 @@ def create(
 
         To create an array with no filters, provide an empty iterable or the value `None`.
     cache_metadata : bool, optional
-        If True, array configuration metadata will be cached for the
-        lifetime of the object. If False, array metadata will be reloaded
-        prior to all data access and modification operations (may incur
-        overhead depending on storage and data access pattern).
+        Ignored. Deprecated and will be removed in a release after April 2027.
     cache_attrs : bool, optional
-        If True (default), user attributes will be cached for attribute read
-        operations. If False, user attributes are reloaded from the store prior
-        to all attribute read operations.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     read_only : bool, optional
-        True if array should be protected against modification.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     object_codec : Codec, optional
-        A codec to encode object arrays, only needed if dtype=object.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     dimension_separator : {'.', '/'}, optional
         Separator placed between the dimensions of a chunk.
         Zarr format 2 only. Zarr format 3 arrays should use `chunk_key_encoding` instead.
@@ -760,7 +746,7 @@ def create(
         which instructs Zarr to choose the default Zarr format value defined in the
         runtime configuration.
     meta_array : array-like, optional
-        Not implemented.
+        Ignored. Deprecated and will be removed in a release after April 2027.
     attributes : dict[str, JSON], optional
         A dictionary of user attributes to store with the array.
     chunk_shape : ChunksLike, optional

diff --git a/tests/test_api.py b/tests/test_api.py
@@ -2013,8 +2013,26 @@ def test_auto_chunks(f: Callable[..., AnyArray]) -> None:
     assert a.chunks == (500, 500)
 
 
-@pytest.mark.parametrize("kwarg_name", ["synchronizer", "chunk_store", "cache_attrs", "meta_array"])
-def test_unimplemented_kwarg_warnings(kwarg_name: str) -> None:
+@pytest.mark.parametrize(
+    "kwarg_name",
+    [
+        "synchronizer",
+        "chunk_store",
+        "cache_metadata",
+        "cache_attrs",
+        "object_codec",
+        "read_only",
+        "meta_array",
+    ],
+)
+def test_ignored_kwarg_warnings(kwarg_name: str) -> None:
     kwargs = {kwarg_name: 1}
-    with pytest.warns(RuntimeWarning, match=".* is not yet implemented"):
+    with pytest.warns(ZarrDeprecationWarning, match=f"{kwarg_name} is ignored"):
         zarr.create(shape=(1,), **kwargs)  # type: ignore[arg-type]
+
+
+@pytest.mark.parametrize("kwarg_name", ["synchronizer", "chunk_store", "cache_attrs", "meta_array"])
+def test_ignored_kwarg_warnings_open_group(kwarg_name: str) -> None:
+    kwargs = {kwarg_name: 1}
+    with pytest.warns(ZarrDeprecationWarning, match=f"{kwarg_name} is ignored"):
+        zarr.open_group(store=MemoryStore(), **kwargs)  # type: ignore[arg-type]
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		The Zarr-Python 2 keyword arguments `synchronizer`, `chunk_store`, `cache_metadata`, `cache_attrs`, `object_codec`, `read_only` and `meta_array` accepted by `zarr.create`, `zarr.group` and `zarr.open_group` (and the functions that wrap them, such as `zarr.zeros`) have never had any effect in Zarr-Python 3. Passing one of them now emits a `ZarrDeprecationWarning` instead of a "not yet implemented" `ZarrRuntimeWarning`. They will be removed in a release after April 2027. Calls that do not pass these arguments are unaffected.