fix: deep-copy source attributes in from_array

changes/4325.bugfix.md

@@ -0,0 +1 @@
+`zarr.from_array` now deep-copies the source array's attributes instead of sharing nested dicts and lists between the source and the new array. Previously, mutating a nested attribute on the new array (for example ``dst.attrs["meta"]["tags"].append(...)``) silently changed the source array's in-memory attributes too.

src/zarr/core/array.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import copy
 import math
 import warnings
 from asyncio import gather
@@ -4955,7 +4956,9 @@ def _parse_keep_array_attr(
         if dimension_names is None and data.metadata.zarr_format == 3:
             dimension_names = data.metadata.dimension_names
         if attributes is None:
-            attributes = dict(data.attrs)
+            # Deep copy so nested containers are not shared between the source
+            # array's in-memory metadata and the new array's.
+            attributes = copy.deepcopy(dict(data.attrs))
     else:
         if chunks == "keep":
             chunks = "auto"

tests/test_array.py

@@ -1829,7 +1829,7 @@ async def test_from_array_arraylike(
 @pytest.mark.parametrize("store", ["local", "memory"], indirect=True)
 def test_from_array_keeps_fill_value_and_attributes(store: Store, zarr_format: ZarrFormat) -> None:
     """`from_array` defaults to the fill value and attributes of the source array."""
-    attributes: dict[str, JSON] = {"units": "K"}
+    attributes: dict[str, JSON] = {"units": "K", "nested": {"x": [1]}, "tags": ["a"]}
     src = zarr.create_array(
         store,
         name="src",
@@ -1845,6 +1845,18 @@ def test_from_array_keeps_fill_value_and_attributes(store: Store, zarr_format: Z
     assert result.fill_value == 42
     assert dict(result.attrs) == attributes
 
+    # The copied attributes must not alias the source's nested containers.
+    nested = result.attrs["nested"]
+    assert isinstance(nested, dict)
+    nested_x = nested["x"]
+    assert isinstance(nested_x, list)
+    nested_x.append(99)
+    tags = result.attrs["tags"]
+    assert isinstance(tags, list)
+    tags.append("b")
+    assert src.attrs["nested"] == {"x": [1]}
+    assert src.attrs["tags"] == ["a"]
+
     # A metadata-only copy must read back the source's fill value, not the dtype default.
     meta_only = zarr.from_array({}, data=src, write_data=False)
     np.testing.assert_array_equal(meta_only[:], np.full((4,), 42, dtype="int32"))