BUG: make order of index from pd.concat deterministic

closes pandas-dev#17344

doc/source/whatsnew/v0.21.0.txt

@@ -405,6 +405,7 @@ Reshaping
 - Bug in :func:`crosstab` where passing two ``Series`` with the same name raised a ``KeyError`` (:issue:`13279`)
 - :func:`Series.argmin`, :func:`Series.argmax`, and their counterparts on ``DataFrame`` and groupby objects work correctly with floating point data that contains infinite values (:issue:`13595`).
 - Bug in :func:`unique` where checking a tuple of strings raised a ``TypeError`` (:issue:`17108`)
+- Bug in :func:`concat` which would not respect the order of the index along the common dimension (:issue:`17344`)
 
 Numeric
 ^^^^^^^

pandas/core/common.py

@@ -629,3 +629,17 @@ def _random_state(state=None):
     else:
         raise ValueError("random_state must be an integer, a numpy "
                          "RandomState, or None")
+
+
+def _get_distinct_objs(objs):
+    """
+    Return a list with distinct elements of "objs" (different ids).
+    Preserves order.
+    """
+    ids = set()
+    res = []
+    for obj in objs:
+        if not id(obj) in ids:
+            ids.add(id(obj))
+            res.append(obj)
+    return res

pandas/core/indexes/api.py

@@ -23,8 +23,7 @@
            'PeriodIndex', 'DatetimeIndex',
            '_new_Index', 'NaT',
            '_ensure_index', '_get_na_value', '_get_combined_index',
-           '_get_objs_combined_axis',
-           '_get_distinct_indexes', '_union_indexes',
+           '_get_objs_combined_axis', '_union_indexes',
            '_get_consensus_names',
            '_all_indexes_same']
 
@@ -41,7 +40,7 @@ def _get_objs_combined_axis(objs, intersect=False, axis=0):
 
 def _get_combined_index(indexes, intersect=False):
     # TODO: handle index names!
-    indexes = _get_distinct_indexes(indexes)
+    indexes = com._get_distinct_objs(indexes)
     if len(indexes) == 0:
         return Index([])
     if len(indexes) == 1:
@@ -55,10 +54,6 @@ def _get_combined_index(indexes, intersect=False):
     return _ensure_index(union)
 
 
-def _get_distinct_indexes(indexes):
-    return list(dict((id(x), x) for x in indexes).values())
-
-
 def _union_indexes(indexes):
     if len(indexes) == 0:
         raise AssertionError('Must have at least 1 Index to union')

pandas/tests/reshape/test_concat.py

@@ -1944,6 +1944,15 @@ def test_concat_categoricalindex(self):
                            index=exp_idx)
         tm.assert_frame_equal(result, exp)
 
+    def test_concat_order(self):
+        # GH 17344
+        dfs = [pd.DataFrame(index=range(3), columns=['a', 1, None])]
+        dfs += [pd.DataFrame(index=range(3), columns=[None, 1, 'a'])
+                for i in range(100)]
+        result = pd.concat(dfs).columns
+        expected = dfs[0].columns
+        tm.assert_index_equal(result, expected)
+
 
 @pytest.mark.parametrize('pdt', [pd.Series, pd.DataFrame, pd.Panel])
 @pytest.mark.parametrize('dt', np.sctypes['float'])