BUG: Concatenation warning still appears with sort=False (#27702)

Author: Nico Cernek

doc/source/whatsnew/v0.25.1.rst

@@ -125,7 +125,7 @@ Reshaping
 ^^^^^^^^^
 
 - A ``KeyError`` is now raised if ``.unstack()`` is called on a :class:`Series` or :class:`DataFrame` with a flat :class:`Index` passing a name which is not the correct one (:issue:`18303`)
--
+- :meth:`DataFrame.join` now suppresses the ``FutureWarning`` when the sort parameter is specified (:issue:`21952`)
 -
 
 Sparse

pandas/core/frame.py

@@ -7210,10 +7210,14 @@ def _join_compat(
             # join indexes only using concat
             if can_concat:
                 if how == "left":
-                    res = concat(frames, axis=1, join="outer", verify_integrity=True)
+                    res = concat(
+                        frames, axis=1, join="outer", verify_integrity=True, sort=sort
+                    )
                     return res.reindex(self.index, copy=False)
                 else:
-                    return concat(frames, axis=1, join=how, verify_integrity=True)
+                    return concat(
+                        frames, axis=1, join=how, verify_integrity=True, sort=sort
+                    )
 
             joined = frames[0]
 

pandas/tests/frame/test_join.py

@@ -193,3 +193,32 @@ def test_join_left_sequence_non_unique_index():
     )
 
     tm.assert_frame_equal(joined, expected)
+
+
+@pytest.mark.parametrize("sort_kw", [True, False, None])
+def test_suppress_future_warning_with_sort_kw(sort_kw):
+    a = DataFrame({"col1": [1, 2]}, index=["c", "a"])
+
+    b = DataFrame({"col2": [4, 5]}, index=["b", "a"])
+
+    c = DataFrame({"col3": [7, 8]}, index=["a", "b"])
+
+    expected = DataFrame(
+        {
+            "col1": {"a": 2.0, "b": float("nan"), "c": 1.0},
+            "col2": {"a": 5.0, "b": 4.0, "c": float("nan")},
+            "col3": {"a": 7.0, "b": 8.0, "c": float("nan")},
+        }
+    )
+    if sort_kw is False:
+        expected = expected.reindex(index=["c", "a", "b"])
+
+    if sort_kw is None:
+        # only warn if not explicitly specified
+        ctx = tm.assert_produces_warning(FutureWarning, check_stacklevel=False)
+    else:
+        ctx = tm.assert_produces_warning(None, check_stacklevel=False)
+
+    with ctx:
+        result = a.join([b, c], how="outer", sort=sort_kw)
+    tm.assert_frame_equal(result, expected)