Fix concat not respecting order of OrderedDict

doc/source/whatsnew/v0.25.0.rst

@@ -241,6 +241,7 @@ Reshaping
 - Bug in :func:`pandas.merge` adds a string of ``None`` if ``None`` is assigned in suffixes instead of remain the column name as-is (:issue:`24782`).
 - Bug in :func:`merge` when merging by index name would sometimes result in an incorrectly numbered index (:issue:`24212`)
 - :func:`to_records` now accepts dtypes to its `column_dtypes` parameter (:issue:`24895`)
+- Bug in :func:`concat` where order of ``OrderedDict`` (and ``dict`` in Python 3.6+) is not respected, when passed in as  ``objs`` argument (:issue:`21510`)
 
 
 Sparse

pandas/core/groupby/generic.py

@@ -822,7 +822,7 @@ def _aggregate_multiple_funcs(self, arg, _level):
                     columns.append(com.get_callable_name(f))
             arg = lzip(columns, arg)
 
-        results = {}
+        results = collections.OrderedDict()
         for name, func in arg:
             obj = self
             if name in results:

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/groupby/test_groupby.py

@@ -1690,9 +1690,9 @@ def test_groupby_agg_ohlc_non_first():
         [1, 1, 1, 1, 1],
         [1, 1, 1, 1, 1]
     ], columns=pd.MultiIndex.from_tuples((
-        ('foo', 'ohlc', 'open'), ('foo', 'ohlc', 'high'),
-        ('foo', 'ohlc', 'low'), ('foo', 'ohlc', 'close'),
-        ('foo', 'sum', 'foo'))), index=pd.date_range(
+        ('foo', 'sum', 'foo'), ('foo', 'ohlc', 'open'),
+        ('foo', 'ohlc', 'high'), ('foo', 'ohlc', 'low'),
+        ('foo', 'ohlc', 'close'))), index=pd.date_range(
             '2018-01-01', periods=2, freq='D'))
 
     result = df.groupby(pd.Grouper(freq='D')).agg(['sum', 'ohlc'])

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
@@ -18,6 +18,7 @@
 from pandas import (
     Categorical, DataFrame, DatetimeIndex, Index, MultiIndex, Series,
     Timestamp, concat, date_range, isna, read_csv)
+import pandas.core.common as com
 from pandas.tests.extension.decimal import to_decimal
 from pandas.util import testing as tm
 from pandas.util.testing import assert_frame_equal, makeCustomDataframe as mkdf
@@ -1162,7 +1163,7 @@ def test_concat_dict(self):
                   'baz': DataFrame(np.random.randn(4, 3)),
                   'qux': DataFrame(np.random.randn(4, 3))}
 
-        sorted_keys = sorted(frames)
+        sorted_keys = com.dict_keys_to_ordered_list(frames)
 
         result = concat(frames)
         expected = concat([frames[k] for k in sorted_keys], keys=sorted_keys)
@@ -2370,6 +2371,14 @@ 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])
 @pytest.mark.parametrize('dt', np.sctypes['float'])