Self review

Author: aldenks

src/zarr/codecs/sharding.py

@@ -778,7 +778,7 @@ async def _load_partial_shard_maybe(
     ) -> ShardMapping | None:
         """
         Read chunks from `byte_getter` for the case where the read is less than a full shard.
-        Returns a mapping of chunk coordinates to bytes.
+        Returns a mapping of chunk coordinates to bytes or None.
         """
         shard_index = await self._load_shard_index_maybe(byte_getter, chunks_per_shard)
         if shard_index is None:
@@ -788,11 +788,9 @@ async def _load_partial_shard_maybe(
             _ChunkCoordsByteSlice(chunk_coords, slice(*chunk_byte_slice))
             for chunk_coords in all_chunk_coords
             # Drop chunks where index lookup fails
-            # e.g. when write_empty_chunks = False and the chunk is empty
+            # e.g. empty chunks when write_empty_chunks = False
             if (chunk_byte_slice := shard_index.get_chunk_slice(chunk_coords))
         ]
-        if len(chunks) == 0:
-            return None
 
         groups = self._coalesce_chunks(chunks)
 
@@ -816,7 +814,7 @@ def _coalesce_chunks(
     ) -> list[list[_ChunkCoordsByteSlice]]:
         """
         Combine chunks from a single shard into groups that should be read together
-        in a single request.
+        in a single request to the store.
 
         Respects the following configuration options:
         - `sharding.read.coalesce_max_gap_bytes`: The maximum gap between
@@ -828,6 +826,9 @@ def _coalesce_chunks(
 
         sorted_chunks = sorted(chunks, key=lambda c: c.byte_slice.start)
 
+        if len(sorted_chunks) == 0:
+            return []
+
         groups = []
         current_group = [sorted_chunks[0]]
 

tests/test_codecs/test_sharding.py

@@ -354,7 +354,7 @@ def test_sharding_read_empty_chunks_within_non_empty_shard_write_empty_false(
         - some, but not all, chunks in the last shard are empty
         - the last shard is not complete (array length is not a multiple of shard shape),
           this takes us down the partial shard read path
-        - write_empty_chunks=False so the shard index will have less entries than chunks in the shard
+        - write_empty_chunks=False so the shard index will have fewer entries than chunks in the shard
     """
     # array with mixed empty and non-empty chunks in second shard
     data = np.array([