TST: Add corrwith index tests (#24519)

Author: Daniel Saxton

pandas/tests/frame/test_analytics.py

@@ -459,6 +459,26 @@ def test_corrwith_mixed_dtypes(self):
         expected = pd.Series(data=corrs, index=['a', 'b'])
         tm.assert_series_equal(result, expected)
 
+    def test_corrwith_index_intersection(self):
+        df1 = pd.DataFrame(np.random.random(size=(10, 2)),
+                           columns=["a", "b"])
+        df2 = pd.DataFrame(np.random.random(size=(10, 3)),
+                           columns=["a", "b", "c"])
+
+        result = df1.corrwith(df2, drop=True).index.sort_values()
+        expected = df1.columns.intersection(df2.columns).sort_values()
+        tm.assert_index_equal(result, expected)
+
+    def test_corrwith_index_union(self):
+        df1 = pd.DataFrame(np.random.random(size=(10, 2)),
+                           columns=["a", "b"])
+        df2 = pd.DataFrame(np.random.random(size=(10, 3)),
+                           columns=["a", "b", "c"])
+
+        result = df1.corrwith(df2, drop=False).index.sort_values()
+        expected = df1.columns.union(df2.columns).sort_values()
+        tm.assert_index_equal(result, expected)
+
     def test_corrwith_dup_cols(self):
         # GH 21925
         df1 = pd.DataFrame(np.vstack([np.arange(10)] * 3).T)