Fix concat not respecting order of OrderedDict

pandas/core/common.py

@@ -14,7 +14,7 @@
 
 from pandas._libs import lib, tslibs
 import pandas.compat as compat
-from pandas.compat import PY36, iteritems
+from pandas.compat import PY37, iteritems
 
 from pandas.core.dtypes.cast import construct_1d_object_array_from_listlike
 from pandas.core.dtypes.common import (
@@ -226,7 +226,7 @@ def try_sort(iterable):
 def dict_keys_to_ordered_list(mapping):
     # when pandas drops support for Python < 3.6, this function
     # can be replaced by a simple list(mapping.keys())
-    if PY36 or isinstance(mapping, OrderedDict):
+    if PY37 or isinstance(mapping, OrderedDict):
         keys = list(mapping.keys())
     else:
         keys = try_sort(mapping)

pandas/core/reshape/concat.py

@@ -253,7 +253,7 @@ def __init__(self, objs, axis=0, join='outer', join_axes=None,
 
         if isinstance(objs, dict):
             if keys is None:
-                keys = sorted(objs)
+                keys = com.dict_keys_to_ordered_list(objs)
             objs = [objs[k] for k in keys]
         else:
             objs = list(objs)

pandas/tests/frame/test_constructors.py

@@ -12,7 +12,8 @@
 import pytest
 
 from pandas.compat import (
-    PY3, PY36, is_platform_little_endian, lmap, long, lrange, lzip, range, zip)
+    PY3, PY36, PY37, is_platform_little_endian, lmap, long, lrange, lzip,
+    range, zip)
 
 from pandas.core.dtypes.cast import construct_1d_object_array_from_listlike
 from pandas.core.dtypes.common import is_integer_dtype
@@ -340,7 +341,7 @@ def test_constructor_dict_nan_tuple_key(self, value):
         result = DataFrame(data, index=idx, columns=cols)
         tm.assert_frame_equal(result, expected)
 
-    @pytest.mark.skipif(not PY36, reason='Insertion order for Python>=3.6')
+    @pytest.mark.skipif(not PY37, reason='Insertion order for Python>=3.6')
     def test_constructor_dict_order_insertion(self):
         # GH19018
         # initialization ordering: by insertion order if python>= 3.6

pandas/tests/reshape/test_concat.py

@@ -1,4 +1,4 @@
-from collections import deque
+from collections import OrderedDict, deque
 import datetime as dt
 from datetime import datetime
 from decimal import Decimal
@@ -2399,6 +2399,14 @@ def test_concat_different_extension_dtypes_upcasts(self):
         ], dtype=object)
         tm.assert_series_equal(result, expected)
 
+    def test_concat_odered_dict(self):
+        # GH 21510
+        expected = pd.concat([pd.Series(range(3)), pd.Series(range(4))],
+                             keys=['First', 'Another'])
+        result = pd.concat(OrderedDict([('First', pd.Series(range(3))),
+                                        ('Another', pd.Series(range(4)))]))
+        tm.assert_series_equal(result, expected)
+
 
 @pytest.mark.parametrize('pdt', [pd.Series, pd.DataFrame, pd.Panel])
 @pytest.mark.parametrize('dt', np.sctypes['float'])

pandas/tests/sparse/frame/test_frame.py

@@ -129,7 +129,7 @@ def test_constructor_dict_order(self):
         # order by value
         d = {'b': [2, 3], 'a': [0, 1]}
         frame = SparseDataFrame(data=d)
-        if compat.PY36:
+        if compat.PY37:
             expected = SparseDataFrame(data=d, columns=list('ba'))
         else:
             expected = SparseDataFrame(data=d, columns=list('ab'))