TST: add np.ndarray and dict guard tests for DataFrame.at/iat list-like setitem (GH#61223)

Author: tinezivic

pandas/tests/indexing/test_at.py

@@ -362,3 +362,35 @@ def test_at_setitem_list_like_multiindex_row(self):
         df.at[("r1", 0), "A"] = [10, 20]
         assert df.at[("r1", 0), "A"] == [10, 20]
         assert df.at[("r2", 1), "A"] == 2
+
+    def test_at_setitem_ndarray_new_column(self):
+        # GH#61223: np.ndarray should be stored as a single cell in a new column
+        df = DataFrame({"A": [1, 2, 3]}, index=["a", "b", "c"])
+        arr = np.array([10, 20, 30])
+        df.at["a", "B"] = arr
+        stored = df.at["a", "B"]
+        assert isinstance(stored, np.ndarray)
+        tm.assert_numpy_array_equal(stored, arr)
+        assert df["B"].dtype == np.dtype("object")
+        assert df.at["b", "B"] is np.nan
+
+    def test_at_setitem_ndarray_existing_column(self):
+        # GH#61223: np.ndarray should be stored as a single cell even when the
+        # existing column has a non-object dtype (column is cast to object)
+        df = DataFrame({"A": [1, 2, 3]}, index=["a", "b", "c"])
+        arr = np.array([100, 200])
+        df.at["b", "A"] = arr
+        stored = df.at["b", "A"]
+        assert isinstance(stored, np.ndarray)
+        tm.assert_numpy_array_equal(stored, arr)
+        assert df["A"].dtype == np.dtype("object")
+        assert df.at["a", "A"] == 1
+        assert df.at["c", "A"] == 3
+
+    def test_at_setitem_dict_value_not_affected(self):
+        # GH#61223: dict values are excluded from the list-like single-cell path;
+        # assigning a dict via .at should still raise ValueError as before
+        # (existing behaviour must not change)
+        df = DataFrame({"A": [1, 2, 3]}, index=["a", "b", "c"])
+        with pytest.raises(ValueError, match="Incompatible indexer"):
+            df.at["a", "A"] = {"key": "val"}