Checking that the store is an instance of dict seem incorrect.

Indeed MemoryStore is an instance of mutable mapping that have the same
property as dict, but woudl not be seen as as instance of dict.

zarr/core.py

@@ -5,6 +5,7 @@
 import operator
 import re
 from functools import reduce
+from collections.abc import MutableMapping
 
 import numpy as np
 from numcodecs.compat import ensure_bytes, ensure_ndarray
@@ -1821,7 +1822,7 @@ def _encode_chunk(self, chunk):
             cdata = chunk
 
         # ensure in-memory data is immutable and easy to compare
-        if isinstance(self.chunk_store, dict):
+        if isinstance(self.chunk_store, MutableMapping):
             cdata = ensure_bytes(cdata)
 
         return cdata

zarr/storage.py

@@ -176,7 +176,7 @@ def getsize(store, path: Path = None) -> int:
     if hasattr(store, 'getsize'):
         # pass through
         return store.getsize(path)
-    elif isinstance(store, dict):
+    elif isinstance(store, MutableMapping):
         # compute from size of values
         if path in store:
             v = store[path]