Revert "Backport PR #24916: BUG-24212 fix regression in #24897 (#24951)"

This reverts commit 84056c5.

doc/source/whatsnew/v0.24.1.rst

@@ -63,9 +63,6 @@ Bug Fixes
 -
 -
 
-**Reshaping**
-
-- Bug in :func:`merge` when merging by index name would sometimes result in an incorrectly numbered index (:issue:`24212`)
 
 **Other**
 

pandas/core/reshape/merge.py

@@ -757,21 +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,
-                                                         right_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,
-                                                         left_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))
@@ -782,39 +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,
-                           other_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()
-                other_list = other_index.take(other_indexer).to_numpy()
-                join_list[mask] = other_list[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

@@ -939,22 +939,25 @@ def test_merge_two_empty_df_no_division_error(self):
         with np.errstate(divide='raise'):
             merge(a, a, on=('a', 'b'))
 
-    @pytest.mark.parametrize('how', ['right', 'outer'])
+    @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 [0, 1, 2, -1, -1, -1] as left_indexer, ensure that
-        # -1 is interpreted as a missing value instead of the last element
-        df1 = pd.DataFrame({'a': [1, 2, 3], 'key': [0, 2, 2]})
-        df2 = pd.DataFrame({'b': [1, 2, 3, 4, 5]})
-        result = df1.merge(df2, left_on='key', right_index=True, how=how)
-        expected = pd.DataFrame([[1.0, 0, 1],
-                                 [2.0, 2, 3],
-                                 [3.0, 2, 3],
-                                 [np.nan, 1, 2],
-                                 [np.nan, 3, 4],
-                                 [np.nan, 4, 5]],
-                                columns=['a', 'key', 'b'])
-        expected.set_index(Int64Index([0, 1, 2, 1, 3, 4]), inplace=True)
+        # pd.merge gets [-1, -1, 0, 1] as right_indexer, ensure that -1 is
+        # interpreted as a missing value instead of the last element
+        df1 = pd.DataFrame([[1, 2], [2, 4], [3, 6], [4, 8]],
+                           columns=['a', 'b'])
+        df2 = pd.DataFrame([[3, 30], [4, 40]],
+                           columns=['a', 'c'])
+        df1.set_index('a', drop=False, inplace=True)
+        df2.set_index('a', inplace=True)
+        result = pd.merge(df1, df2, left_index=True, right_on='a', how=how)
+        expected = pd.DataFrame([[1, 2, np.nan],
+                                 [2, 4, np.nan],
+                                 [3, 6, 30.0],
+                                 [4, 8, 40.0]],
+                                columns=['a', 'b', 'c'])
+        expected.set_index('a', drop=False, inplace=True)
         assert_frame_equal(result, expected)
 
     def test_merge_right_index_right(self):