BUG: DataFrame.join inconsistently setting result index name (#56948)

* Index.join result name

* whatsnew

* update test

doc/source/whatsnew/v3.0.0.rst

@@ -113,8 +113,9 @@ Performance improvements
 
 Bug fixes
 ~~~~~~~~~
+- Fixed bug in :meth:`DataFrame.join` inconsistently setting result index name (:issue:`55815`)
 - Fixed bug in :meth:`Series.diff` allowing non-integer values for the ``periods`` argument. (:issue:`56607`)
-
+-
 
 Categorical
 ^^^^^^^^^^^

pandas/core/indexes/base.py

@@ -4711,6 +4711,10 @@ def _join_via_get_indexer(
                 except TypeError:
                     pass
 
+        names = other.names if how == "right" else self.names
+        if join_index.names != names:
+            join_index = join_index.set_names(names)
+
         if join_index is self:
             lindexer = None
         else:
@@ -5017,7 +5021,7 @@ def _join_monotonic(
                 ridx = self._left_indexer_unique(other)
             else:
                 join_array, lidx, ridx = self._left_indexer(other)
-                join_index = self._wrap_joined_index(join_array, other, lidx, ridx)
+                join_index = self._wrap_joined_index(join_array, other, lidx, ridx, how)
         elif how == "right":
             if self.is_unique:
                 # We can perform much better than the general case
@@ -5026,13 +5030,13 @@ def _join_monotonic(
                 ridx = None
             else:
                 join_array, ridx, lidx = other._left_indexer(self)
-                join_index = self._wrap_joined_index(join_array, other, lidx, ridx)
+                join_index = self._wrap_joined_index(join_array, other, lidx, ridx, how)
         elif how == "inner":
             join_array, lidx, ridx = self._inner_indexer(other)
-            join_index = self._wrap_joined_index(join_array, other, lidx, ridx)
+            join_index = self._wrap_joined_index(join_array, other, lidx, ridx, how)
         elif how == "outer":
             join_array, lidx, ridx = self._outer_indexer(other)
-            join_index = self._wrap_joined_index(join_array, other, lidx, ridx)
+            join_index = self._wrap_joined_index(join_array, other, lidx, ridx, how)
 
         lidx = None if lidx is None else ensure_platform_int(lidx)
         ridx = None if ridx is None else ensure_platform_int(ridx)
@@ -5044,21 +5048,22 @@ def _wrap_joined_index(
         other: Self,
         lidx: npt.NDArray[np.intp],
         ridx: npt.NDArray[np.intp],
+        how: JoinHow,
     ) -> Self:
         assert other.dtype == self.dtype
-
+        names = other.names if how == "right" else self.names
         if isinstance(self, ABCMultiIndex):
-            name = self.names if self.names == other.names else None
             # error: Incompatible return value type (got "MultiIndex",
             # expected "Self")
             mask = lidx == -1
             join_idx = self.take(lidx)
             right = cast("MultiIndex", other.take(ridx))
             join_index = join_idx.putmask(mask, right)._sort_levels_monotonic()
-            return join_index.set_names(name)  # type: ignore[return-value]
+            return join_index.set_names(names)  # type: ignore[return-value]
         else:
-            name = get_op_result_name(self, other)
-            return self._constructor._with_infer(joined, name=name, dtype=self.dtype)
+            return self._constructor._with_infer(
+                joined, name=names[0], dtype=self.dtype
+            )
 
     @final
     @cache_readonly

pandas/core/indexes/datetimelike.py

@@ -75,6 +75,7 @@
 
     from pandas._typing import (
         Axis,
+        JoinHow,
         Self,
         npt,
     )
@@ -735,10 +736,15 @@ def _get_join_freq(self, other):
         return freq
 
     def _wrap_joined_index(
-        self, joined, other, lidx: npt.NDArray[np.intp], ridx: npt.NDArray[np.intp]
+        self,
+        joined,
+        other,
+        lidx: npt.NDArray[np.intp],
+        ridx: npt.NDArray[np.intp],
+        how: JoinHow,
     ):
         assert other.dtype == self.dtype, (other.dtype, self.dtype)
-        result = super()._wrap_joined_index(joined, other, lidx, ridx)
+        result = super()._wrap_joined_index(joined, other, lidx, ridx, how)
         result._data._freq = self._get_join_freq(other)
         return result
 

pandas/tests/indexes/numeric/test_join.py

@@ -21,20 +21,14 @@ def test_join_non_unique(self):
         tm.assert_numpy_array_equal(ridx, exp_ridx)
 
     def test_join_inner(self):
-        index = Index(range(0, 20, 2), dtype=np.int64)
-        other = Index([7, 12, 25, 1, 2, 5], dtype=np.int64)
-        other_mono = Index([1, 2, 5, 7, 12, 25], dtype=np.int64)
+        index = Index(range(0, 20, 2), dtype=np.int64, name="lhs")
+        other = Index([7, 12, 25, 1, 2, 5], dtype=np.int64, name="rhs")
+        other_mono = Index([1, 2, 5, 7, 12, 25], dtype=np.int64, name="rhs")
 
         # not monotonic
         res, lidx, ridx = index.join(other, how="inner", return_indexers=True)
 
-        # no guarantee of sortedness, so sort for comparison purposes
-        ind = res.argsort()
-        res = res.take(ind)
-        lidx = lidx.take(ind)
-        ridx = ridx.take(ind)
-
-        eres = Index([2, 12], dtype=np.int64)
+        eres = Index([2, 12], dtype=np.int64, name="lhs")
         elidx = np.array([1, 6], dtype=np.intp)
         eridx = np.array([4, 1], dtype=np.intp)
 
@@ -46,7 +40,7 @@ def test_join_inner(self):
         # monotonic
         res, lidx, ridx = index.join(other_mono, how="inner", return_indexers=True)
 
-        res2 = index.intersection(other_mono)
+        res2 = index.intersection(other_mono).set_names(["lhs"])
         tm.assert_index_equal(res, res2)
 
         elidx = np.array([1, 6], dtype=np.intp)
@@ -57,9 +51,9 @@ def test_join_inner(self):
         tm.assert_numpy_array_equal(ridx, eridx)
 
     def test_join_left(self):
-        index = Index(range(0, 20, 2), dtype=np.int64)
-        other = Index([7, 12, 25, 1, 2, 5], dtype=np.int64)
-        other_mono = Index([1, 2, 5, 7, 12, 25], dtype=np.int64)
+        index = Index(range(0, 20, 2), dtype=np.int64, name="lhs")
+        other = Index([7, 12, 25, 1, 2, 5], dtype=np.int64, name="rhs")
+        other_mono = Index([1, 2, 5, 7, 12, 25], dtype=np.int64, name="rhs")
 
         # not monotonic
         res, lidx, ridx = index.join(other, how="left", return_indexers=True)
@@ -80,20 +74,20 @@ def test_join_left(self):
         tm.assert_numpy_array_equal(ridx, eridx)
 
         # non-unique
-        idx = Index([1, 1, 2, 5])
-        idx2 = Index([1, 2, 5, 7, 9])
+        idx = Index([1, 1, 2, 5], name="rhs")
+        idx2 = Index([1, 2, 5, 7, 9], name="lhs")
         res, lidx, ridx = idx2.join(idx, how="left", return_indexers=True)
-        eres = Index([1, 1, 2, 5, 7, 9])  # 1 is in idx2, so it should be x2
+        eres = Index([1, 1, 2, 5, 7, 9], name="lhs")  # 1 is in idx2, so it should be x2
         eridx = np.array([0, 1, 2, 3, -1, -1], dtype=np.intp)
         elidx = np.array([0, 0, 1, 2, 3, 4], dtype=np.intp)
         tm.assert_index_equal(res, eres)
         tm.assert_numpy_array_equal(lidx, elidx)
         tm.assert_numpy_array_equal(ridx, eridx)
 
     def test_join_right(self):
-        index = Index(range(0, 20, 2), dtype=np.int64)
-        other = Index([7, 12, 25, 1, 2, 5], dtype=np.int64)
-        other_mono = Index([1, 2, 5, 7, 12, 25], dtype=np.int64)
+        index = Index(range(0, 20, 2), dtype=np.int64, name="lhs")
+        other = Index([7, 12, 25, 1, 2, 5], dtype=np.int64, name="rhs")
+        other_mono = Index([1, 2, 5, 7, 12, 25], dtype=np.int64, name="rhs")
 
         # not monotonic
         res, lidx, ridx = index.join(other, how="right", return_indexers=True)
@@ -115,10 +109,10 @@ def test_join_right(self):
         assert ridx is None
 
         # non-unique
-        idx = Index([1, 1, 2, 5])
-        idx2 = Index([1, 2, 5, 7, 9])
+        idx = Index([1, 1, 2, 5], name="lhs")
+        idx2 = Index([1, 2, 5, 7, 9], name="rhs")
         res, lidx, ridx = idx.join(idx2, how="right", return_indexers=True)
-        eres = Index([1, 1, 2, 5, 7, 9])  # 1 is in idx2, so it should be x2
+        eres = Index([1, 1, 2, 5, 7, 9], name="rhs")  # 1 is in idx2, so it should be x2
         elidx = np.array([0, 1, 2, 3, -1, -1], dtype=np.intp)
         eridx = np.array([0, 0, 1, 2, 3, 4], dtype=np.intp)
         tm.assert_index_equal(res, eres)