Ensure that store_dict used for empty dicts (zarr-developers#2162)

* Ensure that store_dict used for empty dicts

The `store_dict or {}` is Falsey for empty dicts, so the user provided
dict wasn't being used.

* fixup

---------

Co-authored-by: David Stansby <[email protected]>

Author: TomAugspurger

src/zarr/store/memory.py

@@ -30,7 +30,9 @@ def __init__(
         mode: AccessModeLiteral = "r",
     ):
         super().__init__(mode=mode)
-        self._store_dict = store_dict or {}
+        if store_dict is None:
+            store_dict = {}
+        self._store_dict = store_dict
 
     async def empty(self) -> bool:
         return not self._store_dict

tests/v3/test_store/test_memory.py

@@ -18,11 +18,14 @@ def set(self, store: MemoryStore, key: str, value: Buffer) -> None:
     def get(self, store: MemoryStore, key: str) -> Buffer:
         return store._store_dict[key]
 
-    @pytest.fixture(scope="function", params=[None, {}])
+    @pytest.fixture(scope="function", params=[None, True])
     def store_kwargs(
         self, request: pytest.FixtureRequest
     ) -> dict[str, str | None | dict[str, Buffer]]:
-        return {"store_dict": request.param, "mode": "r+"}
+        kwargs = {"store_dict": None, "mode": "r+"}
+        if request.param is True:
+            kwargs["store_dict"] = {}
+        return kwargs
 
     @pytest.fixture(scope="function")
     def store(self, store_kwargs: str | None | dict[str, Buffer]) -> MemoryStore:
@@ -77,3 +80,9 @@ def test_store_supports_partial_writes(self, store: GpuMemoryStore) -> None:
 
     def test_list_prefix(self, store: GpuMemoryStore) -> None:
         assert True
+
+
+def test_uses_dict():
+    store_dict = {}
+    store = MemoryStore(store_dict)
+    assert store._store_dict is store_dict