Delegate logic for chunks to AsyncArray._create

Author: dstansby

changes/xxxx.bugfix.rst

@@ -0,0 +1,7 @@
+When creating arrays without explicitly specifying a chunk size using `zarr.create` and other
+array creation routines, the chunk size will now set automatically instead of defaulting to the data shape.
+For large arrays this will result in smaller default chunk sizes.
+To retain previous behaviour, explicitly set the chunk shape to the data shape.
+
+This fix matches the existing chunking behaviour of
+`zarr.save_array` and `zarr.api.asynchronous.AsyncArray.create`.

src/zarr/api/asynchronous.py

@@ -992,19 +992,11 @@ async def create(
     )
 
     if zarr_format == 2:
-        if chunks is None:
-            chunks = shape
         dtype = parse_dtype(dtype, zarr_format)
         if not filters:
             filters = _default_filters(dtype)
         if compressor == "auto":
             compressor = _default_compressor(dtype)
-    elif zarr_format == 3 and chunk_shape is None:  # type: ignore[redundant-expr]
-        if chunks is not None:
-            chunk_shape = chunks
-            chunks = None
-        else:
-            chunk_shape = shape
 
     if synchronizer is not None:
         warnings.warn("synchronizer is not yet implemented", RuntimeWarning, stacklevel=2)

src/zarr/core/chunk_grids.py

@@ -64,6 +64,9 @@ def _guess_chunks(
     if isinstance(shape, int):
         shape = (shape,)
 
+    if typesize == 0:
+        return shape
+
     ndims = len(shape)
     # require chunks to have non-zero length for all dimensions
     chunks = np.maximum(np.array(shape, dtype="=f8"), 1)

tests/test_api.py

@@ -1301,3 +1301,9 @@ def test_no_overwrite_load(tmp_path: Path) -> None:
     with contextlib.suppress(NotImplementedError):
         zarr.load(store)
     assert existing_fpath.exists()
+
+
+def test_auto_chunks() -> None:
+    # Check chunks are automatically set to a sensible default
+    a = zarr.ones(shape=(1000, 1000), dtype=np.uint8)
+    assert a.chunks == (500, 500)