From 4379a66e24707568c5a84af0548b941612e163d9 Mon Sep 17 00:00:00 2001
From: Davis Vann Bennett <davis.v.bennett@gmail.com>
Date: Thu, 15 Jan 2026 15:25:38 +0100
Subject: [PATCH 1/2] fix partial nested shard reads

---
 src/zarr/codecs/sharding.py        | 13 +++++++--
 tests/test_codecs/test_sharding.py | 47 ++++++++++++++----------------
 2 files changed, 32 insertions(+), 28 deletions(-)

diff --git a/src/zarr/codecs/sharding.py b/src/zarr/codecs/sharding.py
index 8124ea44ea..b54b3c2257 100644
--- a/src/zarr/codecs/sharding.py
+++ b/src/zarr/codecs/sharding.py
@@ -52,6 +52,7 @@
 )
 from zarr.core.metadata.v3 import parse_codecs
 from zarr.registry import get_ndbuffer_class, get_pipeline_class
+from zarr.storage._utils import _normalize_byte_range_index
 
 if TYPE_CHECKING:
     from collections.abc import Iterator
@@ -86,11 +87,16 @@ class _ShardingByteGetter(ByteGetter):
     async def get(
         self, prototype: BufferPrototype, byte_range: ByteRequest | None = None
     ) -> Buffer | None:
-        assert byte_range is None, "byte_range is not supported within shards"
         assert prototype == default_buffer_prototype(), (
             f"prototype is not supported within shards currently. diff: {prototype} != {default_buffer_prototype()}"
         )
-        return self.shard_dict.get(self.chunk_coords)
+        value = self.shard_dict.get(self.chunk_coords)
+        if value is None:
+            return None
+        if byte_range is None:
+            return value
+        start, stop = _normalize_byte_range_index(value, byte_range)
+        return value[start:stop]
 
 
 @dataclass(frozen=True)
@@ -597,7 +603,8 @@ async def _decode_shard_index(
                 )
             )
         )
-        assert index_array is not None
+        # This cannot be None because we have the bytes already
+        index_array = cast(NDBuffer, index_array)
         return _ShardIndex(index_array.as_numpy_array())
 
     async def _encode_shard_index(self, index: _ShardIndex) -> Buffer:
diff --git a/tests/test_codecs/test_sharding.py b/tests/test_codecs/test_sharding.py
index 7eb4deccbf..d0e2d09b7c 100644
--- a/tests/test_codecs/test_sharding.py
+++ b/tests/test_codecs/test_sharding.py
@@ -18,7 +18,6 @@
     TransposeCodec,
 )
 from zarr.core.buffer import NDArrayLike, default_buffer_prototype
-from zarr.errors import ZarrUserWarning
 from zarr.storage import StorePath, ZipStore
 
 from ..conftest import ArrayRequest
@@ -239,12 +238,14 @@ def test_sharding_partial_overwrite(
     assert np.array_equal(data, read_data)
 
 
+# Zip storage raises a warning about a duplicate name, which we ignore.
+@pytest.mark.filterwarnings("ignore:Duplicate name.*:UserWarning")
 @pytest.mark.parametrize(
     "array_fixture",
     [
-        ArrayRequest(shape=(128,) * 3, dtype="uint16", order="F"),
+        ArrayRequest(shape=(127, 128, 129), dtype="uint16", order="F"),
     ],
-    indirect=["array_fixture"],
+    indirect=True,
 )
 @pytest.mark.parametrize(
     "outer_index_location",
@@ -263,24 +264,23 @@ def test_nested_sharding(
 ) -> None:
     data = array_fixture
     spath = StorePath(store)
-    msg = "Combining a `sharding_indexed` codec disables partial reads and writes, which may lead to inefficient performance."
-    with pytest.warns(ZarrUserWarning, match=msg):
-        a = zarr.create_array(
-            spath,
-            shape=data.shape,
-            chunks=(64, 64, 64),
-            dtype=data.dtype,
-            fill_value=0,
-            serializer=ShardingCodec(
-                chunk_shape=(32, 32, 32),
-                codecs=[
-                    ShardingCodec(chunk_shape=(16, 16, 16), index_location=inner_index_location)
-                ],
-                index_location=outer_index_location,
-            ),
-        )
+    # compressors=None ensures no BytesBytesCodec is added, which keeps
+    # supports_partial_decode=True and exercises the partial decode path
+    a = zarr.create_array(
+        spath,
+        data=data,
+        chunks=(64,) * data.ndim,
+        compressors=None,
+        serializer=ShardingCodec(
+            chunk_shape=(32,) * data.ndim,
+            codecs=[
+                ShardingCodec(chunk_shape=(16,) * data.ndim, index_location=inner_index_location)
+            ],
+            index_location=outer_index_location,
+        ),
+    )
 
-    a[:, :, :] = data
+    a[:] = data
 
     read_data = a[0 : data.shape[0], 0 : data.shape[1], 0 : data.shape[2]]
     assert isinstance(read_data, NDArrayLike)
@@ -326,13 +326,10 @@ def test_nested_sharding_create_array(
         filters=None,
         compressors=None,
     )
-    print(a.metadata.to_dict())
 
-    a[:, :, :] = data
+    a[:] = data
 
-    read_data = a[0 : data.shape[0], 0 : data.shape[1], 0 : data.shape[2]]
-    assert isinstance(read_data, NDArrayLike)
-    assert data.shape == read_data.shape
+    read_data = a[:]
     assert np.array_equal(data, read_data)
 
 

From 0282f300d5c387244f9776abb0d70926dcad6c16 Mon Sep 17 00:00:00 2001
From: Davis Vann Bennett <davis.v.bennett@gmail.com>
Date: Thu, 15 Jan 2026 16:17:25 +0100
Subject: [PATCH 2/2] changelog

---
 changes/3655.bugfix.md | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changes/3655.bugfix.md

diff --git a/changes/3655.bugfix.md b/changes/3655.bugfix.md
new file mode 100644
index 0000000000..67d384f00d
--- /dev/null
+++ b/changes/3655.bugfix.md
@@ -0,0 +1 @@
+Fixed a bug in the sharding codec that prevented nested shard reads in certain cases.
\ No newline at end of file