BUG: Index with right_index=True or left_index=True merge

pandas/core/reshape/merge.py

@@ -438,8 +438,8 @@ def merge_asof(
     >>> pd.merge_asof(left, right, left_index=True, right_index=True)
        left_val  right_val
     1         a          1
-    5         b          3
-    10        c          7
+    3         b          3
+    7         c          7
 
     Here is a real-world times-series example
 
@@ -906,23 +906,21 @@ 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,
+                        self.left.index,
                         right_indexer,
-                        how="right",
+                        how="left",
                     )
                 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,
+                        self.right.index,
                         left_indexer,
-                        how="left",
+                        how="right",
                     )
                 else:
                     join_index = self.left.index.take(left_indexer)
@@ -939,7 +937,6 @@ def _create_join_index(
         index: Index,
         other_index: Index,
         indexer,
-        other_indexer,
         how: str = "left",
     ):
         """

pandas/tests/reshape/merge/test_merge.py

@@ -15,6 +15,7 @@
     DataFrame,
     DatetimeIndex,
     Float64Index,
+    Index,
     Int64Index,
     IntervalIndex,
     MultiIndex,
@@ -360,7 +361,9 @@ def test_handle_join_key_pass_array(self):
 
         key = np.array([0, 1, 1, 2, 2, 3], dtype=np.int64)
         merged = merge(left, right, left_index=True, right_on=key, how="outer")
-        tm.assert_series_equal(merged["key_0"], Series(key, name="key_0"))
+        tm.assert_series_equal(
+            merged["key_0"], Series(key, name="key_0", index=[0, 1, 1, 2, 2, np.nan])
+        )
 
     def test_no_overlap_more_informative_error(self):
         dt = datetime.now()
@@ -435,7 +438,7 @@ def test_merge_left_empty_right_empty(self, join_type, kwarg):
 
         exp_in = DataFrame(
             columns=["a", "b", "c", "x", "y", "z"],
-            index=pd.Index([], dtype=object),
+            index=Index([], dtype=object),
             dtype=object,
         )
 
@@ -471,7 +474,10 @@ def check1(exp, kwarg):
         def check2(exp, kwarg):
             result = pd.merge(left, right, how="right", **kwarg)
             tm.assert_frame_equal(result, exp)
+
+        def check3(exp, kwarg, index):
             result = pd.merge(left, right, how="outer", **kwarg)
+            exp.index = index
             tm.assert_frame_equal(result, exp)
 
         for kwarg in [
@@ -481,6 +487,13 @@ def check2(exp, kwarg):
             check1(exp_in, kwarg)
             check2(exp_out, kwarg)
 
+        check3(exp_out, dict(left_index=True, right_index=True), exp_out.index)
+        check3(
+            exp_out.copy(),
+            dict(left_index=True, right_on="x"),
+            Index([np.nan, np.nan, np.nan]),
+        )
+
         kwarg = dict(left_on="a", right_index=True)
         check1(exp_in, kwarg)
         exp_out["a"] = [0, 1, 2]
@@ -1311,7 +1324,7 @@ def test_merge_on_index_with_more_values(self, how, index, expected_index):
             ],
             columns=["a", "key", "b"],
         )
-        expected.set_index(expected_index, inplace=True)
+        expected.set_index(df2.index, inplace=True)
         tm.assert_frame_equal(result, expected)
 
     def test_merge_right_index_right(self):
@@ -1324,7 +1337,7 @@ def test_merge_right_index_right(self):
         expected = DataFrame(
             {"a": [1, 2, 3, None], "key": [0, 1, 1, 2], "b": [1, 2, 2, 3]},
             columns=["a", "key", "b"],
-            index=[0, 1, 2, np.nan],
+            index=[0, 1, 1, 2],
         )
         result = left.merge(right, left_on="key", right_index=True, how="right")
         tm.assert_frame_equal(result, expected)
@@ -1357,7 +1370,7 @@ def test_merge_take_missing_values_from_index_of_other_dtype(self):
                 "key": Categorical(["a", "a", "b", "c"]),
                 "b": [1, 1, 2, 3],
             },
-            index=[0, 1, 2, np.nan],
+            index=Categorical(["a", "a", "b", "c"]),
         )
         expected = expected.reindex(columns=["a", "key", "b"])
         tm.assert_frame_equal(result, expected)
@@ -2349,3 +2362,16 @@ def test_merge_join_cols_error_reporting_on_and_index(func, kwargs):
     )
     with pytest.raises(MergeError, match=msg):
         getattr(pd, func)(left, right, on="a", **kwargs)
+
+
+@pytest.mark.parametrize(
+    ("index", "how", "values"),
+    [(Index(["a", "b"]), "left", ["a", "b"]), (Index([0, 1]), "right", ["a", "c"])],
+)
+def test_left_index_true_left_and_righ_join_target_index(index, how, values):
+    left = DataFrame(index=["a", "b"])
+    right = DataFrame({"x": ["a", "c"]})
+
+    result = merge(left, right, left_index=True, right_on="x", how=how)
+    expected = DataFrame({"x": values}, index=index)
+    tm.assert_frame_equal(result, expected)

pandas/tests/reshape/merge/test_merge_index_as_string.py

@@ -185,5 +185,8 @@ def test_join_indexes_and_columns_on(df1, df2, left_index, join_type):
     result = left_df.join(
         right_df, on=["outer", "inner"], how=join_type, lsuffix="_x", rsuffix="_y"
     )
-
+    if join_type == "right" and left_index == "inner":
+        result.index = result.index.droplevel("outer")
+    if join_type == "outer" and left_index == "inner":
+        result.index = result.index.droplevel(0)
     tm.assert_frame_equal(result, expected, check_like=True)