BUG: fix StringArray/PandasArray setitem with slice

doc/source/whatsnew/v1.0.2.rst

@@ -32,6 +32,10 @@ Bug fixes
 
 - Using ``pd.NA`` with :meth:`DataFrame.to_json` now correctly outputs a null value instead of an empty object (:issue:`31615`)
 
+**Experimental dtypes**
+
+- Fixed bug in setting values using a slice indexer with string dtype (:issue:`31772`)
+
 .. ---------------------------------------------------------------------------
 
 .. _whatsnew_102.contributors:

pandas/core/arrays/numpy_.py

@@ -263,12 +263,8 @@ def __setitem__(self, key, value) -> None:
         value = extract_array(value, extract_numpy=True)
 
         key = check_array_indexer(self, key)
-        scalar_key = lib.is_scalar(key)
         scalar_value = lib.is_scalar(value)
 
-        if not scalar_key and scalar_value:
-            key = np.asarray(key)
-
         if not scalar_value:
             value = np.asarray(value, dtype=self._ndarray.dtype)
 

pandas/tests/extension/base/setitem.py

@@ -173,6 +173,29 @@ def test_setitem_tuple_index(self, data):
         s[(0, 1)] = data[1]
         self.assert_series_equal(s, expected)
 
+    def test_setitem_slice(self, data, box_in_series):
+        arr = data[:5].copy()
+        expected = data.take([0, 0, 0, 3, 4])
+        if box_in_series:
+            arr = pd.Series(arr)
+            expected = pd.Series(expected)
+
+        arr[:3] = data[0]
+        self.assert_equal(arr, expected)
+
+    def test_setitem_loc_iloc_slice(self, data):
+        arr = data[:5].copy()
+        s = pd.Series(arr, index=["a", "b", "c", "d", "e"])
+        expected = pd.Series(data.take([0, 0, 0, 3, 4]), index=s.index)
+
+        result = s.copy()
+        result.iloc[:3] = data[0]
+        self.assert_equal(result, expected)
+
+        result = s.copy()
+        result.loc[:"c"] = data[0]
+        self.assert_equal(result, expected)
+
     def test_setitem_slice_mismatch_length_raises(self, data):
         arr = data[:5]
         with pytest.raises(ValueError):

pandas/tests/extension/test_numpy.py

@@ -396,6 +396,14 @@ def test_setitem_scalar_key_sequence_raise(self, data):
         # Failed: DID NOT RAISE <class 'ValueError'>
         super().test_setitem_scalar_key_sequence_raise(data)
 
+    @skip_nested
+    def test_setitem_slice(self, data, box_in_series):
+        super().test_setitem_slice(data, box_in_series)
+
+    @skip_nested
+    def test_setitem_loc_iloc_slice(self, data):
+        super().test_setitem_loc_iloc_slice(data)
+
 
 @skip_nested
 class TestParsing(BaseNumPyTests, base.BaseParsingTests):