BUG - pd.concat with all Series on axis=1 ignores the names argument (#23499)

Closes gh-23490.

Author: ms7463

doc/source/whatsnew/v0.24.0.txt

@@ -1316,6 +1316,7 @@ Reshaping
 ^^^^^^^^^
 
 - Bug in :func:`pandas.concat` when joining resampled DataFrames with timezone aware index (:issue:`13783`)
+- Bug in :func:`pandas.concat` when joining only `Series` the `names` argument of `concat` is no longer ignored (:issue:`23490`)
 - Bug in :meth:`Series.combine_first` with ``datetime64[ns, tz]`` dtype which would return tz-naive result (:issue:`21469`)
 - Bug in :meth:`Series.where` and :meth:`DataFrame.where` with ``datetime64[ns, tz]`` dtype (:issue:`21546`)
 - Bug in :meth:`DataFrame.where` with an empty DataFrame and empty ``cond`` having non-bool dtype (:issue:`21947`)

pandas/core/reshape/concat.py

@@ -502,7 +502,7 @@ def _get_concat_axis(self):
                 else:
                     return ibase.default_index(len(self.objs))
             else:
-                return ensure_index(self.keys)
+                return ensure_index(self.keys).set_names(self.names)
         else:
             indexes = [x._data.axes[self.axis] for x in self.objs]
 

pandas/tests/reshape/test_concat.py

@@ -1618,6 +1618,23 @@ def test_concat_series_axis1(self, sort=sort):
         expected = DataFrame({'A': s, 'B': s2})
         assert_frame_equal(result, expected)
 
+    def test_concat_series_axis1_names_applied(self):
+        # ensure names argument is not ignored on axis=1, #23490
+        s = Series([1, 2, 3])
+        s2 = Series([4, 5, 6])
+        result = concat([s, s2], axis=1, keys=['a', 'b'], names=['A'])
+        expected = DataFrame([[1, 4], [2, 5], [3, 6]],
+                             columns=pd.Index(['a', 'b'], name='A'))
+        assert_frame_equal(result, expected)
+
+        result = concat([s, s2], axis=1, keys=[('a', 1), ('b', 2)],
+                        names=['A', 'B'])
+        expected = DataFrame([[1, 4], [2, 5], [3, 6]],
+                             columns=MultiIndex.from_tuples([('a', 1),
+                                                             ('b', 2)],
+                                                            names=['A', 'B']))
+        assert_frame_equal(result, expected)
+
     def test_concat_single_with_key(self):
         df = DataFrame(np.random.randn(10, 4))