BUG: GH10355 groupby std() doesnt sqrt grouping cols

doc/source/whatsnew/v0.17.1.txt

@@ -75,6 +75,7 @@ Bug Fixes
 - Bug in merging ``datetime64[ns, tz]`` dtypes (:issue:`11405`)
 - Bug in ``HDFStore.select`` when comparing with a numpy scalar in a where clause (:issue:`11283`)
 - Bug in using ``DataFrame.ix`` with a multi-index indexer(:issue:`11372`)
+- Bug in ``groupby.var()`` when using ``DataFrame.groupby`` with as_index=False (:issue:`10355`)
 
 
 - Bug in tz-conversions with an ambiguous time and ``.dt`` accessors (:issue:`11295`)

pandas/core/groupby.py

@@ -789,7 +789,25 @@ 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 self.as_index:
+            return np.sqrt(self.var(ddof=ddof))
+        else:
+            df = self.var(ddof=ddof)
+
+            # if we are selecting columns we musn't root them
+            try:
+                if type(self.keys) in (list, tuple) and all(k in df.columns for
+                                                            k in self.keys):
+                    to_root = [c for c in df.columns if c not in self.keys]
+                elif self.keys in df.columns:
+                    to_root = [c for c in df.columns if c != self.keys]
+                else:
+                    return np.sqrt(df)
+            except TypeError:
+                return np.sqrt(df)
+
+            df[to_root] = np.sqrt(df[to_root])
+            return df
 
     def var(self, ddof=1):
         """

pandas/tests/test_groupby.py

@@ -5546,6 +5546,60 @@ 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_and_column_grouping(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()
+        sd2 = df.groupby(['a'], as_index=False).std()
+        sd3 = df.groupby(('a'), as_index=False).std()
+
+        expected = pd.DataFrame({
+            'a': [1, 2, 3],
+            'b': [1.0, 1.0, 1.0],
+        })
+        tm.assert_frame_equal(expected, sd)
+        tm.assert_frame_equal(expected, sd2)
+        tm.assert_frame_equal(expected, sd3)
+
+    def test_std_with_as_index_false_and_index_grouping(self):
+        df = pd.DataFrame({
+            'a': [1, 1, 1, 2, 2, 2, 3, 4, 5],
+            'b': [1, 2, 3, 4, 5, 6, 7, 8, 9],
+        })
+        sd = df.groupby(lambda x: 1+int(x/3), as_index=False).std()
+        sd2 = df.groupby([1, 1, 1, 2, 2, 2, 3, 3, 3], as_index=False).std()
+        s = pd.Series([1, 1, 1, 2, 2, 2, 3, 3, 3])
+        sd3 = df.groupby(s, as_index=False).std()
+        sd4 = df.groupby(dict(s), as_index=False).std()
+
+        expected = pd.DataFrame({
+            'a': [0.0, 0.0, 1.0],
+            'b': [1.0, 1.0, 1.0],
+        })
+        tm.assert_frame_equal(expected, sd)
+        tm.assert_frame_equal(expected, sd2)
+        tm.assert_frame_equal(expected, sd3)
+        tm.assert_frame_equal(expected, sd4)
+
+    def test_std_with_as_index_false_and_ddof_zero(self):
+        df = pd.DataFrame({
+            1: [1, 1, 1, 2, 2, 2, 3, 3, 3],
+            2: [1, 2, 3, 1, 5, 6, 7, 8, 10],
+        })
+        sd = df.groupby(1, as_index=False).std(ddof=0)
+
+        expected = pd.DataFrame({
+            1: [1, 2, 3],
+            2: [
+                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()