From 6f141c19ae8dad2ffad55a4645e4784a8bdb34fa Mon Sep 17 00:00:00 2001
From: Davis Vann Bennett <davis.v.bennett@gmail.com>
Date: Wed, 9 Sep 2026 12:45:05 +0200
Subject: [PATCH 1/2] fix: deep-copy source attributes in from_array

`from_array` copied the source array's attributes with `dict(data.attrs)`,
a shallow copy, so any nested dict or list inside the attributes was shared
between the source array's in-memory metadata and the new array's. Mutating
a nested attribute on the copy (e.g. appending to a list) silently changed
the source array's attributes as well. The alias never reached the source's
store, but it was created by zarr without the user's involvement.

Copy the attributes with `copy.deepcopy` instead. Attributes are JSON values
(dicts, lists, scalars), so a deep copy is safe and cheap. The existing
`from_array` attribute test now mutates a nested dict and a nested list on
the copy and asserts the source is unchanged.

Assisted-by: ClaudeCode:claude-fable-5-1
---
 changes/0000.bugfix.md |  1 +
 src/zarr/core/array.py |  5 ++++-
 tests/test_array.py    | 14 +++++++++++++-
 3 files changed, 18 insertions(+), 2 deletions(-)
 create mode 100644 changes/0000.bugfix.md

diff --git a/changes/0000.bugfix.md b/changes/0000.bugfix.md
new file mode 100644
index 0000000000..de5f941e95
--- /dev/null
+++ b/changes/0000.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.
diff --git a/src/zarr/core/array.py b/src/zarr/core/array.py
index bc43802ec7..67075d03c2 100644
--- a/src/zarr/core/array.py
+++ b/src/zarr/core/array.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import copy
 import math
 import warnings
 from asyncio import gather
@@ -4948,7 +4949,9 @@ 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"
diff --git a/tests/test_array.py b/tests/test_array.py
index 46890244ec..a1cd687ab9 100644
--- a/tests/test_array.py
+++ b/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"))

From 8d89e8e9b037795b8902557463c96001996702c6 Mon Sep 17 00:00:00 2001
From: Davis Vann Bennett <davis.v.bennett@gmail.com>
Date: Wed, 9 Sep 2026 12:49:01 +0200
Subject: [PATCH 2/2] chore: rename changelog fragment to the PR number

Assisted-by: ClaudeCode:claude-fable-5-1
---
 changes/{0000.bugfix.md => 4325.bugfix.md} | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 rename changes/{0000.bugfix.md => 4325.bugfix.md} (100%)

diff --git a/changes/0000.bugfix.md b/changes/4325.bugfix.md
similarity index 100%
rename from changes/0000.bugfix.md
rename to changes/4325.bugfix.md