BUG: concat not sorting mixed column names when None is included (#47331)

* REGR: concat not sorting columns for mixed column names

* Fix none in columns

* BUG: concat not sorting column names when None is included

* Update doc/source/whatsnew/v1.5.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* Add gh reference

Co-authored-by: Matthew Roeschke <[email protected]>

Author: phofl

doc/source/whatsnew/v1.5.0.rst

@@ -924,6 +924,7 @@ Reshaping
 - Bug in :func:`get_dummies` that selected object and categorical dtypes but not string (:issue:`44965`)
 - Bug in :meth:`DataFrame.align` when aligning a :class:`MultiIndex` to a :class:`Series` with another :class:`MultiIndex` (:issue:`46001`)
 - Bug in concanenation with ``IntegerDtype``, or ``FloatingDtype`` arrays where the resulting dtype did not mirror the behavior of the non-nullable dtypes (:issue:`46379`)
+- Bug in :func:`concat` not sorting the column names when ``None`` is included (:issue:`47331`)
 - Bug in :func:`concat` with identical key leads to error when indexing :class:`MultiIndex` (:issue:`46519`)
 - Bug in :meth:`DataFrame.join` with a list when using suffixes to join DataFrames with duplicate column names (:issue:`46396`)
 - Bug in :meth:`DataFrame.pivot_table` with ``sort=False`` results in sorted index (:issue:`17041`)

pandas/core/algorithms.py

@@ -1771,9 +1771,12 @@ def safe_sort(
 def _sort_mixed(values) -> np.ndarray:
     """order ints before strings in 1d arrays, safe in py3"""
     str_pos = np.array([isinstance(x, str) for x in values], dtype=bool)
-    nums = np.sort(values[~str_pos])
+    none_pos = np.array([x is None for x in values], dtype=bool)
+    nums = np.sort(values[~str_pos & ~none_pos])
     strs = np.sort(values[str_pos])
-    return np.concatenate([nums, np.asarray(strs, dtype=object)])
+    return np.concatenate(
+        [nums, np.asarray(strs, dtype=object), np.array(values[none_pos])]
+    )
 
 
 def _sort_tuples(values: np.ndarray) -> np.ndarray:

pandas/tests/reshape/concat/test_concat.py

@@ -469,12 +469,12 @@ def __iter__(self):
         tm.assert_frame_equal(concat(CustomIterator2(), ignore_index=True), expected)
 
     def test_concat_order(self):
-        # GH 17344
+        # GH 17344, GH#47331
         dfs = [DataFrame(index=range(3), columns=["a", 1, None])]
-        dfs += [DataFrame(index=range(3), columns=[None, 1, "a"]) for i in range(100)]
+        dfs += [DataFrame(index=range(3), columns=[None, 1, "a"]) for _ in range(100)]
 
         result = concat(dfs, sort=True).columns
-        expected = dfs[0].columns
+        expected = Index([1, "a", None])
         tm.assert_index_equal(result, expected)
 
     def test_concat_different_extension_dtypes_upcasts(self):