API: consistency in aggregate groupby results

closes #12334

Author: Jeff Reback <[email protected]>

Closes #12335 from jreback/groupby2 and squashes the following commits:

522dd79 [Jeff Reback] API: consistency in aggregate groupby results

Author: jreback

doc/source/whatsnew/v0.18.0.txt

@@ -615,7 +615,7 @@ other anchored offsets like ``MonthBegin`` and ``YearBegin``.
 Resample API
 ^^^^^^^^^^^^
 
-Like the change in the window functions API :ref:`above <whatsnew_0180.enhancements.moments>`, ``.resample(...)`` is changing to have a more groupby-like API. (:issue:`11732`, :issue:`12702`, :issue:`12202`, :issue:`12332`, :issue:`12348`).
+Like the change in the window functions API :ref:`above <whatsnew_0180.enhancements.moments>`, ``.resample(...)`` is changing to have a more groupby-like API. (:issue:`11732`, :issue:`12702`, :issue:`12202`, :issue:`12332`, :issue:`12334`, :issue:`12348`).
 
 .. ipython:: python
 

pandas/core/groupby.py

@@ -2550,15 +2550,7 @@ def aggregate(self, func_or_funcs, *args, **kwargs):
         # _level handled at higher
         if not _level and isinstance(ret, dict):
             from pandas import concat
-
-            # our result is a Series-like
-            if len(ret) == 1:
-                ret = concat([r for r in ret.values()],
-                             axis=1)
-
-            # our result is a DataFrame like
-            else:
-                ret = concat(ret, axis=1)
+            ret = concat(ret, axis=1)
         return ret
 
     agg = aggregate

pandas/tests/test_groupby.py

@@ -1532,6 +1532,34 @@ def test_aggregate_api_consistency(self):
                                                     ['D', 'C']])
         assert_frame_equal(result, expected, check_like=True)
 
+    def test_agg_compat(self):
+
+        # GH 12334
+
+        df = DataFrame({'A': ['foo', 'bar', 'foo', 'bar',
+                              'foo', 'bar', 'foo', 'foo'],
+                        'B': ['one', 'one', 'two', 'two',
+                              'two', 'two', 'one', 'two'],
+                        'C': np.random.randn(8) + 1.0,
+                        'D': np.arange(8)})
+
+        g = df.groupby(['A', 'B'])
+
+        expected = pd.concat([g['D'].sum(),
+                              g['D'].std()],
+                             axis=1)
+        expected.columns = MultiIndex.from_tuples([('C', 'sum'),
+                                                   ('C', 'std')])
+        result = g['D'].agg({'C': ['sum', 'std']})
+        assert_frame_equal(result, expected, check_like=True)
+
+        expected = pd.concat([g['D'].sum(),
+                              g['D'].std()],
+                             axis=1)
+        expected.columns = ['C', 'D']
+        result = g['D'].agg({'C': 'sum', 'D': 'std'})
+        assert_frame_equal(result, expected, check_like=True)
+
     def test_agg_nested_dicts(self):
 
         # API change for disallowing these types of nested dicts

pandas/tseries/tests/test_resample.py

@@ -424,8 +424,8 @@ def test_agg_misc(self):
             expected = pd.concat([t['A'].sum(),
                                   t['A'].std()],
                                  axis=1)
-            expected.columns = ['sum', 'std']
-
+            expected.columns = pd.MultiIndex.from_tuples([('A', 'sum'),
+                                                          ('A', 'std')])
             assert_frame_equal(result, expected, check_like=True)
 
             expected = pd.concat([t['A'].agg(['sum', 'std']),