Error on invalid store mode

Author: dstansby

src/zarr/storage/_common.py

@@ -2,7 +2,7 @@
 
 import json
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Literal
+from typing import TYPE_CHECKING, Any, Literal, Self
 
 from zarr.abc.store import ByteRequest, Store
 from zarr.core.buffer import Buffer, default_buffer_prototype
@@ -48,9 +48,7 @@ def read_only(self) -> bool:
         return self.store.read_only
 
     @classmethod
-    async def open(
-        cls, store: Store, path: str, mode: AccessModeLiteral | None = None
-    ) -> StorePath:
+    async def open(cls, store: Store, path: str, mode: AccessModeLiteral | None = None) -> Self:
         """
         Open StorePath based on the provided mode.
 
@@ -67,6 +65,9 @@ async def open(
         ------
         FileExistsError
             If the mode is 'w-' and the store path already exists.
+        ValueError
+            If the mode is not "r" and the store is read-only, or
+            if the mode is "r" and the store is not read-only.
         """
 
         await store._ensure_open()
@@ -78,6 +79,8 @@ async def open(
 
         if store.read_only and mode != "r":
             raise ValueError(f"Store is read-only but mode is '{mode}'")
+        if not store.read_only and mode == "r":
+            raise ValueError(f"Store is not read-only but mode is '{mode}'")
 
         match mode:
             case "w-":

tests/test_store/test_core.py

@@ -4,6 +4,7 @@
 import pytest
 from _pytest.compat import LEGACY_PATH
 
+import zarr
 from zarr import Group
 from zarr.core.common import AccessModeLiteral, ZarrFormat
 from zarr.storage import FsspecStore, LocalStore, MemoryStore, StoreLike, StorePath
@@ -251,3 +252,10 @@ def test_relativize_path_invalid() -> None:
     msg = f"The first component of {path} does not start with {prefix}."
     with pytest.raises(ValueError, match=msg):
         _relativize_path(path="a/b/c", prefix="b")
+
+
+def test_invalid_open_mode() -> None:
+    store = MemoryStore()
+    zarr.create((100,), store=store, zarr_format=2, path="a")
+    with pytest.raises(ValueError, match="Store is not read-only but mode is 'r'"):
+        zarr.open_array(store=store, path="a", zarr_format=2, mode="r")