TST: Fix test assertion (pandas-dev#23357)

Author: TomAugspurger

pandas/tests/extension/base/setitem.py

@@ -27,18 +27,20 @@ def test_setitem_sequence(self, data, box_in_series):
     @pytest.mark.parametrize('as_array', [True, False])
     def test_setitem_sequence_mismatched_length_raises(self, data, as_array):
         ser = pd.Series(data)
+        original = ser.copy()
         value = [data[0]]
         if as_array:
             value = data._from_sequence(value)
 
         xpr = 'cannot set using a {} indexer with a different length'
         with tm.assert_raises_regex(ValueError, xpr.format('list-like')):
             ser[[0, 1]] = value
-            assert ser._values[[0, 1]] == value
+        # Ensure no modifications made before the exception
+        self.assert_series_equal(ser, original)
 
         with tm.assert_raises_regex(ValueError, xpr.format('slice')):
             ser[slice(3)] = value
-            assert ser._values[slice(3)] == value
+        self.assert_series_equal(ser, original)
 
     def test_setitem_empty_indxer(self, data, box_in_series):
         if box_in_series: