Fix zarr.open default for argument mode when store is read_only

changes/3128.bugfix.rst

@@ -0,0 +1 @@
+Fix `zarr.open` default for argument `mode` when `store` is `read_only`

src/zarr/api/asynchronous.py

@@ -9,6 +9,7 @@
 import numpy.typing as npt
 from typing_extensions import deprecated
 
+from zarr.abc.store import Store
 from zarr.core.array import (
     Array,
     AsyncArray,
@@ -289,7 +290,7 @@
 async def open(
     *,
     store: StoreLike | None = None,
-    mode: AccessModeLiteral = "a",
+    mode: AccessModeLiteral | None = None,
     zarr_version: ZarrFormat | None = None,  # deprecated
     zarr_format: ZarrFormat | None = None,
     path: str | None = None,
@@ -324,7 +325,11 @@
         Return type depends on what exists in the given store.
     """
     zarr_format = _handle_zarr_version_or_format(zarr_version=zarr_version, zarr_format=zarr_format)
-
+    if mode is None:
+        if isinstance(store, Store) and store.read_only:
+            mode = "r"
+        else:
+            mode = "a"
     store_path = await make_store_path(store, mode=mode, path=path, storage_options=storage_options)
 
     # TODO: the mode check below seems wrong!

tests/test_api.py

@@ -39,7 +39,7 @@
 )
 from zarr.core.buffer import NDArrayLike
 from zarr.errors import MetadataValidationError
-from zarr.storage import MemoryStore
+from zarr.storage import LocalStore, MemoryStore, ZipStore
 from zarr.storage._utils import normalize_path
 from zarr.testing.utils import gpu_test
 
@@ -400,6 +400,39 @@ def test_load_array(memory_store: Store) -> None:
             assert_array_equal(bar, array)
 
 
+@pytest.mark.parametrize("path", ["data", None])
+def test_load_zip(tmp_path: pathlib.Path, path: str | None) -> None:
+    file = tmp_path / "test.zip"
+    data = np.arange(100).reshape(10, 10)
+
+    with ZipStore(file, mode="w", read_only=False) as zs:
+        save(zs, data, path=path)
+    with ZipStore(file, mode="r", read_only=False) as zs:
+        result = zarr.load(store=zs, path=path)
+        assert np.array_equal(result, data)
+    with ZipStore(file, mode="r") as zs:
+        result = zarr.load(store=zs, path=path)
+        assert_array_equal(result, data)
+    with ZipStore(file, read_only=True) as zs:
+        result = zarr.load(store=zs, path=path)
+        assert_array_equal(result, data)
+
+
+@pytest.mark.parametrize("path", ["data", None])
+def test_load_local(tmp_path: pathlib.Path, path: str | None) -> None:
+    file = tmp_path / "test.zip"
+    data = np.arange(100).reshape(10, 10)
+
+    with LocalStore(file, read_only=False) as zs:
+        save(zs, data, path=path)
+    with LocalStore(file, read_only=False) as zs:
+        result = zarr.load(store=zs, path=path)
+        assert_array_equal(result, data)
+    with LocalStore(file, read_only=True) as zs:
+        result = zarr.load(store=zs, path=path)
+        assert_array_equal(result, data)
+
+
 def test_tree() -> None:
     pytest.importorskip("rich")
     g1 = zarr.group()