fix open_array for mode r+

Author: d-v-b

src/zarr/api/asynchronous.py

@@ -1097,7 +1097,7 @@ async def open_array(
     try:
         return await AsyncArray.open(store_path, zarr_format=zarr_format)
     except FileNotFoundError:
-        if not store_path.read_only:
+        if not store_path.read_only and mode in _CREATE_MODES:
             exists_ok = _infer_exists_ok(mode)
             _zarr_format = zarr_format or _default_zarr_version()
             return await create(

tests/test_api.py

@@ -1030,3 +1030,19 @@ async def test_metadata_validation_error() -> None:
         match="Invalid value for 'zarr_format'. Expected '2, 3, or None'. Got '3.0'.",
     ):
         await zarr.api.asynchronous.open_array(shape=(1,), zarr_format="3.0")  # type: ignore[arg-type]
+
+
+@pytest.mark.parametrize(
+    "store",
+    ["local", "memory", "zip"],
+    indirect=True,
+)
+def test_open_array_with_mode_r_plus(store: Store) -> None:
+    # 'r+' means read/write (must exist)
+    with pytest.raises(FileNotFoundError):
+        zarr.open_array(store=store, mode="r+")
+    zarr.ones(store=store, shape=(3, 3))
+    z2 = zarr.open_array(store=store, mode="r+")
+    assert isinstance(z2, Array)
+    assert (z2[:] == 1).all()
+    z2[:] = 3