fix: let open_like create arrays by default

changes/3352.bugfix.md

@@ -0,0 +1,3 @@
+Fix `zarr.api.asynchronous.open_like` so it can create a new array by default when the
+target path does not already exist. It now defaults to `mode="a"`; when using a read-only
+store to open an existing array, pass `mode="r"` explicitly.

src/zarr/api/asynchronous.py

@@ -1292,14 +1292,18 @@ async def open_like(a: ArrayLike, path: str, **kwargs: Any) -> AnyAsyncArray:
     path : str
         The path to the new array.
     **kwargs
-        Any keyword arguments to pass to the array constructor.
+        Additional keyword arguments passed to `open_array`.
+        If `mode` is omitted or `None`, it defaults to `"a"`. Pass `mode="r"` when
+        opening an existing array from a read-only store.
 
     Returns
     -------
     AsyncArray
         The opened array.
     """
     like_kwargs = _like_args(a) | kwargs
+    if like_kwargs.get("mode") is None:
+        like_kwargs["mode"] = "a"
     return await open_array(path=path, **like_kwargs)  # type: ignore[arg-type]
 
 

src/zarr/api/synchronous.py

@@ -1399,11 +1399,13 @@ def open_like(a: ArrayLike, path: str, **kwargs: Any) -> AnyArray:
     path : str
         The path to the new array.
     **kwargs
-        Any keyword arguments to pass to the array constructor.
+        Additional keyword arguments passed to `open_array`.
+        If `mode` is omitted or `None`, it defaults to `"a"`. Pass `mode="r"` when
+        opening an existing array from a read-only store.
 
     Returns
     -------
-    AsyncArray
+    Array
         The opened array.
     """
     return Array(sync(async_api.open_like(a, path=path, **kwargs)))

tests/test_api.py

@@ -168,6 +168,53 @@ async def test_array_like_creation(
     assert np.all(Array(new_arr)[:] == expect_fill)
 
 
+@pytest.mark.parametrize("mode_kwargs", [{}, {"mode": None}])
+async def test_open_like_creates_array_by_default(
+    zarr_format: ZarrFormat, mode_kwargs: dict[str, None]
+) -> None:
+    ref_arr = zarr.create_array(
+        store={},
+        shape=(11, 12),
+        dtype="uint8",
+        chunks=(11, 12),
+        zarr_format=zarr_format,
+        fill_value=100,
+    )
+
+    new_arr = await zarr.api.asynchronous.open_like(
+        ref_arr,
+        path="foo",
+        store={},
+        zarr_format=zarr_format,
+        **mode_kwargs,
+    )
+
+    assert new_arr.shape == ref_arr.shape
+    assert new_arr.chunks == ref_arr.chunks
+    assert new_arr.dtype == ref_arr.dtype
+    assert np.all(Array(new_arr)[:] == ref_arr.fill_value)
+
+
+async def test_open_like_default_mode_rejects_read_only_store(
+    zarr_format: ZarrFormat,
+) -> None:
+    ref_arr = zarr.create_array(
+        store={},
+        shape=(11, 12),
+        dtype="uint8",
+        chunks=(11, 12),
+        zarr_format=zarr_format,
+    )
+
+    with pytest.raises(ValueError, match="Store is read-only but mode is 'a'"):
+        await zarr.api.asynchronous.open_like(
+            ref_arr,
+            path="foo",
+            store=MemoryStore(read_only=True),
+            zarr_format=zarr_format,
+        )
+
+
 # TODO: parametrize over everything this function takes
 @pytest.mark.parametrize("store", ["memory"], indirect=True)
 def test_create_array(store: Store, zarr_format: ZarrFormat) -> None: