Revert BUG-24212 fix usage of Index.take in pd.merge (#24904)

* Revert BUG-24212 fix usage of Index.take in pd.merge

xref #24733
xref #24897

* test 0.23.4 output

* added note about buggy test

Author: TomAugspurger

doc/source/whatsnew/v0.24.0.rst

@@ -1827,7 +1827,6 @@ Reshaping
 - Bug in :func:`DataFrame.unstack` where a ``ValueError`` was raised when unstacking timezone aware values (:issue:`18338`)
 - Bug in :func:`DataFrame.stack` where timezone aware values were converted to timezone naive values (:issue:`19420`)
 - Bug in :func:`merge_asof` where a ``TypeError`` was raised when ``by_col`` were timezone aware values (:issue:`21184`)
-- Bug in :func:`merge` when merging by index name would sometimes result in an incorrectly numbered index (:issue:`24212`)
 - Bug showing an incorrect shape when throwing error during ``DataFrame`` construction. (:issue:`20742`)
 
 .. _whatsnew_0240.bug_fixes.sparse:

pandas/core/reshape/merge.py

@@ -757,19 +757,13 @@ def _get_join_info(self):
 
             if self.right_index:
                 if len(self.left) > 0:
-                    join_index = self._create_join_index(self.left.index,
-                                                         self.right.index,
-                                                         left_indexer,
-                                                         how='right')
+                    join_index = self.left.index.take(left_indexer)
                 else:
                     join_index = self.right.index.take(right_indexer)
                     left_indexer = np.array([-1] * len(join_index))
             elif self.left_index:
                 if len(self.right) > 0:
-                    join_index = self._create_join_index(self.right.index,
-                                                         self.left.index,
-                                                         right_indexer,
-                                                         how='left')
+                    join_index = self.right.index.take(right_indexer)
                 else:
                     join_index = self.left.index.take(left_indexer)
                     right_indexer = np.array([-1] * len(join_index))
@@ -780,37 +774,6 @@ def _get_join_info(self):
             join_index = join_index.astype(object)
         return join_index, left_indexer, right_indexer
 
-    def _create_join_index(self, index, other_index, indexer, how='left'):
-        """
-        Create a join index by rearranging one index to match another
-
-        Parameters
-        ----------
-        index: Index being rearranged
-        other_index: Index used to supply values not found in index
-        indexer: how to rearrange index
-        how: replacement is only necessary if indexer based on other_index
-
-        Returns
-        -------
-        join_index
-        """
-        join_index = index.take(indexer)
-        if (self.how in (how, 'outer') and
-                not isinstance(other_index, MultiIndex)):
-            # if final index requires values in other_index but not target
-            # index, indexer may hold missing (-1) values, causing Index.take
-            # to take the final value in target index
-            mask = indexer == -1
-            if np.any(mask):
-                # if values missing (-1) from target index,
-                # take from other_index instead
-                join_list = join_index.to_numpy()
-                join_list[mask] = other_index.to_numpy()[mask]
-                join_index = Index(join_list, dtype=join_index.dtype,
-                                   name=join_index.name)
-        return join_index
-
     def _get_merge_keys(self):
         """
         Note: has side effects (copy/delete key columns)

pandas/tests/reshape/merge/test_merge.py

@@ -940,6 +940,7 @@ def test_merge_two_empty_df_no_division_error(self):
             merge(a, a, on=('a', 'b'))
 
     @pytest.mark.parametrize('how', ['left', 'outer'])
+    @pytest.mark.xfail(reason="GH-24897")
     def test_merge_on_index_with_more_values(self, how):
         # GH 24212
         # pd.merge gets [-1, -1, 0, 1] as right_indexer, ensure that -1 is
@@ -959,6 +960,22 @@ def test_merge_on_index_with_more_values(self, how):
         expected.set_index('a', drop=False, inplace=True)
         assert_frame_equal(result, expected)
 
+    def test_merge_right_index_right(self):
+        # Note: the expected output here is probably incorrect.
+        # See https://github.com/pandas-dev/pandas/issues/17257 for more.
+        # We include this as a regression test for GH-24897.
+        left = pd.DataFrame({'a': [1, 2, 3], 'key': [0, 1, 1]})
+        right = pd.DataFrame({'b': [1, 2, 3]})
+
+        expected = pd.DataFrame({'a': [1, 2, 3, None],
+                                 'key': [0, 1, 1, 2],
+                                 'b': [1, 2, 2, 3]},
+                                columns=['a', 'key', 'b'],
+                                index=[0, 1, 2, 2])
+        result = left.merge(right, left_on='key', right_index=True,
+                            how='right')
+        tm.assert_frame_equal(result, expected)
+
 
 def _check_merge(x, y):
     for how in ['inner', 'left', 'outer']: