Deterministic chunk padding

changes/2755.bugfix.rst

@@ -0,0 +1,3 @@
+The array returned by ``zarr.empty`` and an empty ``zarr.core.buffer.cpu.NDBuffer`` will now be filled with the
+specified fill value, or with zeros if no fill value is provided.
+This fixes a bug where Zarr format 2 data with no fill value was written with un-predictable chunk sizes.

src/zarr/api/asynchronous.py

@@ -1065,20 +1065,15 @@ async def create(
 async def empty(
     shape: ChunkCoords, **kwargs: Any
 ) -> AsyncArray[ArrayV2Metadata] | AsyncArray[ArrayV3Metadata]:
-    """Create an empty array.
+    """Create an empty array with the specified shape. The contents will be filled with the
+    array's fill value or zeros if no fill value is provided.
 
     Parameters
     ----------
     shape : int or tuple of int
         Shape of the empty array.
     **kwargs
         Keyword arguments passed to :func:`zarr.api.asynchronous.create`.
-
-    Notes
-    -----
-    The contents of an empty Zarr array are not defined. On attempting to
-    retrieve data from an empty Zarr array, any values may be returned,
-    and these are not guaranteed to be stable from one access to the next.
     """
 
     return await create(shape=shape, fill_value=None, **kwargs)
@@ -1087,7 +1082,8 @@ async def empty(
 async def empty_like(
     a: ArrayLike, **kwargs: Any
 ) -> AsyncArray[ArrayV2Metadata] | AsyncArray[ArrayV3Metadata]:
-    """Create an empty array like `a`.
+    """Create an empty array like `a`. The contents will be filled with the
+    array's fill value or zeros if no fill value is provided.
 
     Parameters
     ----------

src/zarr/api/synchronous.py

@@ -902,7 +902,8 @@ def create_array(
 
 # TODO: add type annotations for kwargs
 def empty(shape: ChunkCoords, **kwargs: Any) -> Array:
-    """Create an empty array.
+    """Create an empty array with the specified shape. The contents will be filled with the
+    array's fill value or zeros if no fill value is provided.
 
     Parameters
     ----------
@@ -915,20 +916,15 @@ def empty(shape: ChunkCoords, **kwargs: Any) -> Array:
     -------
     Array
         The new array.
-
-    Notes
-    -----
-    The contents of an empty Zarr array are not defined. On attempting to
-    retrieve data from an empty Zarr array, any values may be returned,
-    and these are not guaranteed to be stable from one access to the next.
     """
     return Array(sync(async_api.empty(shape, **kwargs)))
 
 
 # TODO: move ArrayLike to common module
 # TODO: add type annotations for kwargs
 def empty_like(a: ArrayLike, **kwargs: Any) -> Array:
-    """Create an empty array like another array.
+    """Create an empty array like another array. The contents will be filled with the
+    array's fill value or zeros if no fill value is provided.
 
     Parameters
     ----------

src/zarr/core/buffer/cpu.py

@@ -154,10 +154,10 @@ def create(
         order: Literal["C", "F"] = "C",
         fill_value: Any | None = None,
     ) -> Self:
-        ret = cls(np.empty(shape=tuple(shape), dtype=dtype, order=order))
-        if fill_value is not None:
-            ret.fill(fill_value)
-        return ret
+        if fill_value is None:
+            return cls(np.zeros(shape=tuple(shape), dtype=dtype, order=order))
+        else:
+            return cls(np.full(shape=tuple(shape), fill_value=fill_value, dtype=dtype, order=order))
 
     @classmethod
     def from_numpy_array(cls, array_like: npt.ArrayLike) -> Self:

src/zarr/core/group.py

@@ -1498,7 +1498,8 @@ async def tree(self, expand: bool | None = None, level: int | None = None) -> An
     async def empty(
         self, *, name: str, shape: ChunkCoords, **kwargs: Any
     ) -> AsyncArray[ArrayV2Metadata] | AsyncArray[ArrayV3Metadata]:
-        """Create an empty array in this Group.
+        """Create an empty array with the specified shape in this Group. The contents will
+        be filled with the array's fill value or zeros if no fill value is provided.
 
         Parameters
         ----------
@@ -1592,7 +1593,8 @@ async def full(
     async def empty_like(
         self, *, name: str, data: async_api.ArrayLike, **kwargs: Any
     ) -> AsyncArray[ArrayV2Metadata] | AsyncArray[ArrayV3Metadata]:
-        """Create an empty sub-array like `data`.
+        """Create an empty sub-array like `data`. The contents will be filled with
+        the array's fill value or zeros if no fill value is provided.
 
         Parameters
         ----------
@@ -2442,7 +2444,8 @@ def require_array(self, name: str, *, shape: ShapeLike, **kwargs: Any) -> Array:
 
     @_deprecate_positional_args
     def empty(self, *, name: str, shape: ChunkCoords, **kwargs: Any) -> Array:
-        """Create an empty array in this Group.
+        """Create an empty array with the specified shape in this Group. The contents will be filled with
+        the array's fill value or zeros if no fill value is provided.
 
         Parameters
         ----------
@@ -2453,11 +2456,6 @@ def empty(self, *, name: str, shape: ChunkCoords, **kwargs: Any) -> Array:
         **kwargs
             Keyword arguments passed to :func:`zarr.api.asynchronous.create`.
 
-        Notes
-        -----
-        The contents of an empty Zarr array are not defined. On attempting to
-        retrieve data from an empty Zarr array, any values may be returned,
-        and these are not guaranteed to be stable from one access to the next.
         """
         return Array(self._sync(self._async_group.empty(name=name, shape=shape, **kwargs)))
 
@@ -2531,7 +2529,8 @@ def full(
 
     @_deprecate_positional_args
     def empty_like(self, *, name: str, data: async_api.ArrayLike, **kwargs: Any) -> Array:
-        """Create an empty sub-array like `data`.
+        """Create an empty sub-array like `data`. The contents will be filled
+        with the array's fill value or zeros if no fill value is provided.
 
         Parameters
         ----------

tests/test_store/test_memory.py

@@ -1,12 +1,19 @@
 from __future__ import annotations
 
+from typing import TYPE_CHECKING
+
+import numpy as np
 import pytest
 
+import zarr
 from zarr.core.buffer import Buffer, cpu, gpu
 from zarr.storage import GpuMemoryStore, MemoryStore
 from zarr.testing.store import StoreTests
 from zarr.testing.utils import gpu_test
 
+if TYPE_CHECKING:
+    from zarr.core.common import ZarrFormat
+
 
 class TestMemoryStore(StoreTests[MemoryStore, cpu.Buffer]):
     store_cls = MemoryStore
@@ -46,6 +53,25 @@ def test_store_supports_partial_writes(self, store: MemoryStore) -> None:
     def test_list_prefix(self, store: MemoryStore) -> None:
         assert True
 
+    @pytest.mark.parametrize("dtype", ["uint8", "float32", "int64"])
+    @pytest.mark.parametrize("zarr_format", [2, 3])
+    async def test_deterministic_size(
+        self, store: MemoryStore, dtype, zarr_format: ZarrFormat
+    ) -> None:
+        a = zarr.empty(
+            store=store,
+            shape=(3,),
+            chunks=(1000,),
+            dtype=dtype,
+            zarr_format=zarr_format,
+            overwrite=True,
+        )
+        a[...] = 1
+        a.resize((1000,))
+
+        np.testing.assert_array_equal(a[:3], 1)
+        np.testing.assert_array_equal(a[3:], 0)
+
 
 @gpu_test
 class TestGpuMemoryStore(StoreTests[GpuMemoryStore, gpu.Buffer]):