ENH: Accept list as level for groupby in non-MultiIndexed objects

closes #13901

Author: agraboso <agraboso@gmail.com>

Closes #13907 from agraboso/fix-13901 and squashes the following commits:

fcbb724 [agraboso] ENH: Accept list as level for groupby in non-MultiIndexed objects

Author: agraboso

doc/source/whatsnew/v0.19.0.txt

@@ -515,6 +515,7 @@ Other enhancements
 - ``astype()`` will now accept a dict of column name to data types mapping as the ``dtype`` argument. (:issue:`12086`)
 - The ``pd.read_json`` and ``DataFrame.to_json`` has gained support for reading and writing json lines with ``lines`` option see :ref:`Line delimited json <io.jsonl>` (:issue:`9180`)
 - :func:``read_excel`` now supports the true_values and false_values keyword arguments (:issue:`13347`)
+- ``groupby()`` will now accept a scalar and a single-element list for specifying ``level`` on a non-``MultiIndex`` grouper. (:issue:`13907`)
 
 .. _whatsnew_0190.api:
 

pandas/core/groupby.py

@@ -23,6 +23,7 @@
                                  is_complex_dtype,
                                  is_bool_dtype,
                                  is_scalar,
+                                 is_list_like,
                                  _ensure_float64,
                                  _ensure_platform_int,
                                  _ensure_int64,
@@ -2370,12 +2371,26 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
     # axis of the object
     if level is not None:
         if not isinstance(group_axis, MultiIndex):
+            # allow level to be a length-one list-like object
+            # (e.g., level=[0])
+            # GH 13901
+            if is_list_like(level):
+                nlevels = len(level)
+                if nlevels == 1:
+                    level = level[0]
+                elif nlevels == 0:
+                    raise ValueError('No group keys passed!')
+                else:
+                    raise ValueError('multiple levels only valid with '
+                                     'MultiIndex')
+
             if isinstance(level, compat.string_types):
                 if obj.index.name != level:
                     raise ValueError('level name %s is not the name of the '
                                      'index' % level)
-            elif level > 0:
-                raise ValueError('level > 0 only valid with MultiIndex')
+            elif level > 0 or level < -1:
+                raise ValueError('level > 0 or level < -1 only valid with '
+                                 ' MultiIndex')
 
             level = None
             key = group_axis

pandas/tests/test_groupby.py

@@ -2029,7 +2029,7 @@ def loop(df):
 
             loop(frame)
 
-    def test_mulitindex_passthru(self):
+    def test_multiindex_passthru(self):
 
         # GH 7997
         # regression from 0.14.1
@@ -2039,6 +2039,24 @@ def test_mulitindex_passthru(self):
         result = df.groupby(axis=1, level=[0, 1]).first()
         assert_frame_equal(result, df)
 
+    def test_multiindex_negative_level(self):
+        # GH 13901
+        result = self.mframe.groupby(level=-1).sum()
+        expected = self.mframe.groupby(level='second').sum()
+        assert_frame_equal(result, expected)
+
+        result = self.mframe.groupby(level=-2).sum()
+        expected = self.mframe.groupby(level='first').sum()
+        assert_frame_equal(result, expected)
+
+        result = self.mframe.groupby(level=[-2, -1]).sum()
+        expected = self.mframe
+        assert_frame_equal(result, expected)
+
+        result = self.mframe.groupby(level=[-1, 'first']).sum()
+        expected = self.mframe.groupby(level=['second', 'first']).sum()
+        assert_frame_equal(result, expected)
+
     def test_multifunc_select_col_integer_cols(self):
         df = self.df
         df.columns = np.arange(len(df.columns))
@@ -2566,13 +2584,28 @@ def test_groupby_level_mapper(self):
         assert_frame_equal(result0, expected0)
         assert_frame_equal(result1, expected1)
 
-    def test_groupby_level_0_nonmulti(self):
-        # #1313
-        a = Series([1, 2, 3, 10, 4, 5, 20, 6], Index([1, 2, 3, 1,
-                                                      4, 5, 2, 6], name='foo'))
+    def test_groupby_level_nonmulti(self):
+        # GH 1313, GH 13901
+        s = Series([1, 2, 3, 10, 4, 5, 20, 6],
+                   Index([1, 2, 3, 1, 4, 5, 2, 6], name='foo'))
+        expected = Series([11, 22, 3, 4, 5, 6],
+                          Index(range(1, 7), name='foo'))
 
-        result = a.groupby(level=0).sum()
-        self.assertEqual(result.index.name, a.index.name)
+        result = s.groupby(level=0).sum()
+        self.assert_series_equal(result, expected)
+        result = s.groupby(level=[0]).sum()
+        self.assert_series_equal(result, expected)
+        result = s.groupby(level=-1).sum()
+        self.assert_series_equal(result, expected)
+        result = s.groupby(level=[-1]).sum()
+        self.assert_series_equal(result, expected)
+
+        tm.assertRaises(ValueError, s.groupby, level=1)
+        tm.assertRaises(ValueError, s.groupby, level=-2)
+        tm.assertRaises(ValueError, s.groupby, level=[])
+        tm.assertRaises(ValueError, s.groupby, level=[0, 0])
+        tm.assertRaises(ValueError, s.groupby, level=[0, 1])
+        tm.assertRaises(ValueError, s.groupby, level=[1])
 
     def test_groupby_complex(self):
         # GH 12902