BUG: DataFrame.join with left or right empty not respecting sort=True (#56443)

* join with empty not respecting sort param

* whatsnew

Author: lukemanley

doc/source/whatsnew/v2.2.0.rst

@@ -247,7 +247,7 @@ These are bug fixes that might have notable behavior changes.
 
 In previous versions of pandas, :func:`merge` and :meth:`DataFrame.join` did not
 always return a result that followed the documented sort behavior. pandas now
-follows the documented sort behavior in merge and join operations (:issue:`54611`, :issue:`56426`).
+follows the documented sort behavior in merge and join operations (:issue:`54611`, :issue:`56426`, :issue:`56443`).
 
 As documented, ``sort=True`` sorts the join keys lexicographically in the resulting
 :class:`DataFrame`. With ``sort=False``, the order of the join keys depends on the

pandas/core/indexes/base.py

@@ -4576,9 +4576,6 @@ def join(
                     pother, how=how, level=level, return_indexers=True, sort=sort
                 )
 
-        lindexer: np.ndarray | None
-        rindexer: np.ndarray | None
-
         # try to figure out the join level
         # GH3662
         if level is None and (self._is_multi or other._is_multi):
@@ -4592,25 +4589,38 @@ def join(
         if level is not None and (self._is_multi or other._is_multi):
             return self._join_level(other, level, how=how)
 
+        lidx: np.ndarray | None
+        ridx: np.ndarray | None
+
         if len(other) == 0:
             if how in ("left", "outer"):
-                join_index = self._view()
-                rindexer = np.broadcast_to(np.intp(-1), len(join_index))
-                return join_index, None, rindexer
+                if sort and not self.is_monotonic_increasing:
+                    lidx = self.argsort()
+                    join_index = self.take(lidx)
+                else:
+                    lidx = None
+                    join_index = self._view()
+                ridx = np.broadcast_to(np.intp(-1), len(join_index))
+                return join_index, lidx, ridx
             elif how in ("right", "inner", "cross"):
                 join_index = other._view()
-                lindexer = np.array([])
-                return join_index, lindexer, None
+                lidx = np.array([], dtype=np.intp)
+                return join_index, lidx, None
 
         if len(self) == 0:
             if how in ("right", "outer"):
-                join_index = other._view()
-                lindexer = np.broadcast_to(np.intp(-1), len(join_index))
-                return join_index, lindexer, None
+                if sort and not other.is_monotonic_increasing:
+                    ridx = other.argsort()
+                    join_index = other.take(ridx)
+                else:
+                    ridx = None
+                    join_index = other._view()
+                lidx = np.broadcast_to(np.intp(-1), len(join_index))
+                return join_index, lidx, ridx
             elif how in ("left", "inner", "cross"):
                 join_index = self._view()
-                rindexer = np.array([])
-                return join_index, None, rindexer
+                ridx = np.array([], dtype=np.intp)
+                return join_index, None, ridx
 
         if self.dtype != other.dtype:
             dtype = self._find_common_type_compat(other)

pandas/tests/frame/methods/test_combine_first.py

@@ -82,7 +82,7 @@ def test_combine_first(self, float_frame, using_infer_string):
         tm.assert_frame_equal(comb, float_frame)
 
         comb = DataFrame().combine_first(float_frame)
-        tm.assert_frame_equal(comb, float_frame)
+        tm.assert_frame_equal(comb, float_frame.sort_index())
 
         comb = float_frame.combine_first(DataFrame(index=["faz", "boo"]))
         assert "faz" in comb.index

pandas/tests/frame/test_arithmetic.py

@@ -629,10 +629,12 @@ def test_arith_flex_frame_corner(self, float_frame):
 
         # corner cases
         result = float_frame.add(float_frame[:0])
-        tm.assert_frame_equal(result, float_frame * np.nan)
+        expected = float_frame.sort_index() * np.nan
+        tm.assert_frame_equal(result, expected)
 
         result = float_frame[:0].add(float_frame)
-        tm.assert_frame_equal(result, float_frame * np.nan)
+        expected = float_frame.sort_index() * np.nan
+        tm.assert_frame_equal(result, expected)
 
         with pytest.raises(NotImplementedError, match="fill_value"):
             float_frame.add(float_frame.iloc[0], fill_value=3)

pandas/tests/reshape/merge/test_join.py

@@ -1023,6 +1023,8 @@ def test_join_empty(left_empty, how, exp):
         expected = DataFrame(columns=["B", "C"], dtype="int64")
         if how != "cross":
             expected = expected.rename_axis("A")
+    if how == "outer":
+        expected = expected.sort_index()
 
     tm.assert_frame_equal(result, expected)
 

pandas/tests/series/test_logical_ops.py

@@ -40,11 +40,11 @@ def test_logical_operators_bool_dtype_with_empty(self):
         s_empty = Series([], dtype=object)
 
         res = s_tft & s_empty
-        expected = s_fff
+        expected = s_fff.sort_index()
         tm.assert_series_equal(res, expected)
 
         res = s_tft | s_empty
-        expected = s_tft
+        expected = s_tft.sort_index()
         tm.assert_series_equal(res, expected)
 
     def test_logical_operators_int_dtype_with_int_dtype(self):
@@ -397,11 +397,11 @@ def test_logical_ops_label_based(self, using_infer_string):
         empty = Series([], dtype=object)
 
         result = a & empty.copy()
-        expected = Series([False, False, False], list("bca"))
+        expected = Series([False, False, False], list("abc"))
         tm.assert_series_equal(result, expected)
 
         result = a | empty.copy()
-        expected = Series([True, False, True], list("bca"))
+        expected = Series([True, True, False], list("abc"))
         tm.assert_series_equal(result, expected)
 
         # vs non-matching