BUG: Fix concat key name

closes pandas-dev#14252
Fixes a bug where `pd.concat` didn't propagate the names of keys used to
create  a hierarchical index.

Author: Ben Kandel <[email protected]>

Closes pandas-dev#14292 from bkandel/fix_concat_key_name and squashes the following commits:

cdc76f6 [Ben Kandel] take out _ensure_index
4a301f8 [Ben Kandel] put back in Index coercion
d8e2c17 [Ben Kandel] remove coercion to Index before _ensure_index
44932cc [Ben Kandel] changed whatsnew entry to 0.19.1
c51df19 [Ben Kandel] _ensure_index
b54b081 [Ben Kandel] simplified logic
3256119 [Ben Kandel] typo
789ecd4 [Ben Kandel] use _ensure_index
350e724 [Ben Kandel] simplified logic
9615a69 [Ben Kandel] extra tests
5c0108b [Ben Kandel] comments
dd3c4cc [Ben Kandel] comments
5cd8392 [Ben Kandel] added test for names
bc5f1fb [Ben Kandel] cleanup
ef6db68 [Ben Kandel] BUG: Propagate key names in concat.

Author: Ben Kandel

doc/source/whatsnew/v0.19.1.txt

@@ -30,3 +30,4 @@ Performance Improvements
 
 Bug Fixes
 ~~~~~~~~~
+- Bug in ``concat`` where names of keys were not propagated to the resulting MultiIndex (:issue:`14252`)

pandas/tests/frame/test_combine_concat.py

@@ -4,21 +4,21 @@
 
 from datetime import datetime
 
-from numpy import nan
 import numpy as np
+from numpy import nan
 
-from pandas.compat import lrange
-from pandas import DataFrame, Series, Index, Timestamp
 import pandas as pd
 
-from pandas.util.testing import (assert_series_equal,
-                                 assert_frame_equal,
-                                 assertRaisesRegexp)
-
-import pandas.util.testing as tm
+from pandas import DataFrame, Index, Series, Timestamp
+from pandas.compat import lrange
 
 from pandas.tests.frame.common import TestData
 
+import pandas.util.testing as tm
+from pandas.util.testing import (assertRaisesRegexp,
+                                 assert_frame_equal,
+                                 assert_series_equal)
+
 
 class TestDataFrameConcatCommon(tm.TestCase, TestData):
 
@@ -324,6 +324,29 @@ def test_join_multiindex_leftright(self):
         assert_frame_equal(df2.join(df1, how='left'),
                            exp[['value2', 'value1']])
 
+    def test_concat_named_keys(self):
+        # GH 14252
+        df = pd.DataFrame({'foo': [1, 2], 'bar': [0.1, 0.2]})
+        index = Index(['a', 'b'], name='baz')
+        concatted_named_from_keys = pd.concat([df, df], keys=index)
+        expected_named = pd.DataFrame(
+            {'foo': [1, 2, 1, 2], 'bar': [0.1, 0.2, 0.1, 0.2]},
+            index=pd.MultiIndex.from_product((['a', 'b'], [0, 1]),
+                                             names=['baz', None]))
+        assert_frame_equal(concatted_named_from_keys, expected_named)
+
+        index_no_name = Index(['a', 'b'], name=None)
+        concatted_named_from_names = pd.concat(
+            [df, df], keys=index_no_name, names=['baz'])
+        assert_frame_equal(concatted_named_from_names, expected_named)
+
+        concatted_unnamed = pd.concat([df, df], keys=index_no_name)
+        expected_unnamed = pd.DataFrame(
+            {'foo': [1, 2, 1, 2], 'bar': [0.1, 0.2, 0.1, 0.2]},
+            index=pd.MultiIndex.from_product((['a', 'b'], [0, 1]),
+                                             names=[None, None]))
+        assert_frame_equal(concatted_unnamed, expected_unnamed)
+
 
 class TestDataFrameCombineFirst(tm.TestCase, TestData):
 

pandas/tools/merge.py

@@ -1369,7 +1369,9 @@ def __init__(self, objs, axis=0, join='outer', join_axes=None,
                 clean_keys.append(k)
                 clean_objs.append(v)
             objs = clean_objs
-            keys = clean_keys
+            name = getattr(keys, 'name', None)
+            keys = Index(clean_keys)
+            keys.name = name
 
         if len(objs) == 0:
             raise ValueError('All objects passed were None')
@@ -1454,7 +1456,7 @@ def __init__(self, objs, axis=0, join='outer', join_axes=None,
         self.axis = axis
         self.join_axes = join_axes
         self.keys = keys
-        self.names = names
+        self.names = names or getattr(keys, 'names', None)
         self.levels = levels
 
         self.ignore_index = ignore_index