TST: Test .loc #25548 for matched and unmatched indices of Series

pandas/tests/indexing/test_loc.py

@@ -3297,3 +3297,22 @@ def test_loc_reindexing_of_empty_index(self):
         df.loc[Series([False] * 4, index=df.index, name=0), 0] = df[0]
         expected = DataFrame(index=[1, 1, 2, 2], data=["1", "1", "2", "2"])
         tm.assert_frame_equal(df, expected)
+
+    def test_loc_setitem_matching_index(self, series_with_simple_index):
+        # GH 25548
+        ser1 = series_with_simple_index.copy()
+        ser1 = ser1[~ser1.index.duplicated(keep="first")]
+        ser2 = ser1.copy()
+        # Testing on upto 2 indices
+        for nsize in range(1, min(len(ser1), 3)):
+            random_sample = ser1.sample(n=nsize)
+            matching_mask = ser1.index.isin(random_sample.index)
+            # Remove values at indices to test assignment
+            ser1.loc[matching_mask] = np.nan
+            ser1.loc[matching_mask] = random_sample
+            tm.assert_series_equal(ser1, ser2)
+            # exclude row and index and test assignment of unmatched indices
+            exclude_mask = ~matching_mask
+            ser2.loc[matching_mask] = ser1.loc[exclude_mask]
+            ser1.loc[matching_mask] = np.nan
+            tm.assert_series_equal(ser2, ser1)