BUG: GH10355 groupby std() no longer sqrts grouping cols

pandas/core/groupby.py

@@ -789,7 +789,12 @@ def std(self, ddof=1):
         For multiple groupings, the result index will be a MultiIndex
         """
         # todo, implement at cython level?
-        return np.sqrt(self.var(ddof=ddof))
+        if ddof == 1:
+            return self._cython_agg_general('std')
+        else:
+            self._set_selection_from_grouper()
+            f = lambda x: x.std(ddof=ddof)
+            return self._python_agg_general(f)
 
     def var(self, ddof=1):
         """
@@ -1467,6 +1472,10 @@ def get_group_levels(self):
     #------------------------------------------------------------
     # Aggregation functions
 
+    def _cython_std(group_var, out, b, c, d):
+        group_var(out, b, c, d)
+        out **= 0.5  # needs to be applied in place
+
     _cython_functions = {
         'add': 'group_add',
         'prod': 'group_prod',
@@ -1477,6 +1486,10 @@ def get_group_levels(self):
             'name': 'group_median'
         },
         'var': 'group_var',
+        'std': {
+            'name': 'group_var',
+            'f': _cython_std,
+        },
         'first': {
             'name': 'group_nth',
             'f': lambda func, a, b, c, d: func(a, b, c, d, 1)
@@ -1512,7 +1525,7 @@ def get_func(fname):
 
             # a sub-function
             f = ftype.get('f')
-            if f is not None:
+            if f is not None and afunc is not None:
 
                 def wrapper(*args, **kwargs):
                     return f(afunc, *args, **kwargs)

pandas/tests/test_groupby.py

@@ -5545,6 +5545,36 @@ def test_nunique_with_object(self):
         expected = pd.Series([1] * 5, name='name', index=index)
         tm.assert_series_equal(result, expected)
 
+    def test_std_with_as_index_false(self):
+        # GH 10355
+        df = pd.DataFrame({
+            'a': [1, 1, 1, 2, 2, 2, 3, 3, 3],
+            'b': [1, 2, 3, 4, 5, 6, 7, 8, 9],
+        })
+        sd = df.groupby('a', as_index=False).std()
+
+        expected = pd.DataFrame({
+            'a': [1, 2, 3],
+            'b': [1, 1, 1],
+        })
+        tm.assert_frame_equal(expected, sd)
+
+    def test_std_with_ddof(self):
+        df = pd.DataFrame({
+            'a': [1, 1, 1, 2, 2, 2, 3, 3, 3],
+            'b': [1, 2, 3, 1, 5, 6, 7, 8, 10],
+        })
+        sd = df.groupby('a', as_index=False).std(ddof=0)
+
+        expected = pd.DataFrame({
+            'a': [1, 2, 3],
+            'b': [
+                np.std([1, 2, 3], ddof=0),
+                np.std([1, 5, 6], ddof=0),
+                np.std([7, 8, 10], ddof=0)],
+        })
+        tm.assert_frame_equal(expected, sd)
+
 
 def assert_fp_equal(a, b):
     assert (np.abs(a - b) < 1e-12).all()