fix: restore attrs del functionality (#2908)

* fix: restore attrs del functionality

* test: fully test

* test: explicitly test both group and array - also reload

* doc: add changelog entry

* test: correct typing

Author: ianhi

Diff for: changes/2908.bugfix.rst

@@ -0,0 +1 @@
+Restore functionality of `del z.attrs['key']` to actually delete the key.

Diff for: src/zarr/core/attributes.py

@@ -28,7 +28,7 @@ def __setitem__(self, key: str, value: JSON) -> None:
     def __delitem__(self, key: str) -> None:
         new_attrs = dict(self._obj.metadata.attributes)
         del new_attrs[key]
-        self._obj = self._obj.update_attributes(new_attrs)
+        self.put(new_attrs)
 
     def __iter__(self) -> Iterator[str]:
         return iter(self._obj.metadata.attributes)

Diff for: tests/test_attributes.py

@@ -1,3 +1,5 @@
+import pytest
+
 import zarr.core
 import zarr.core.attributes
 import zarr.storage
@@ -59,3 +61,24 @@ def test_update_no_changes() -> None:
 
     z.update_attributes({})
     assert dict(z.attrs) == {"a": [], "b": 3}
+
+
+@pytest.mark.parametrize("group", [True, False])
+def test_del_works(group: bool) -> None:
+    store = zarr.storage.MemoryStore()
+    z: zarr.Group | zarr.Array
+    if group:
+        z = zarr.create_group(store)
+    else:
+        z = zarr.create_array(store=store, shape=10, dtype=int)
+    assert dict(z.attrs) == {}
+    z.update_attributes({"a": [3, 4], "c": 4})
+    del z.attrs["a"]
+    assert dict(z.attrs) == {"c": 4}
+
+    z2: zarr.Group | zarr.Array
+    if group:
+        z2 = zarr.open_group(store)
+    else:
+        z2 = zarr.open_array(store)
+    assert dict(z2.attrs) == {"c": 4}