BUG: Fixed bug in groupby.std changing target column when as_index=False

doc/source/whatsnew/v0.17.0.txt

@@ -1067,6 +1067,7 @@ Bug Fixes
 - Bug in ``algos.outer_join_indexer`` when ``right`` array is empty (:issue:`10618`)
 
 - Bug in ``filter`` (regression from 0.16.0) and ``transform`` when grouping on multiple keys, one of which is datetime-like (:issue:`10114`)
+- Bug in ``groupby(as_index=False)`` with ``std`` accidentally modifying target column at the same time (:issue:`10355`)
 
 
 - Bug in ``to_datetime`` and ``to_timedelta`` causing ``Index`` name to be lost (:issue:`10875`)

pandas/core/groupby.py

@@ -788,8 +788,9 @@ 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))
+        self._set_selection_from_grouper()
+        f = lambda x: np.sqrt(x.var(ddof=ddof))
+        return self._python_agg_general(f)
 
     def var(self, ddof=1):
         """

pandas/tests/test_groupby.py

@@ -888,6 +888,20 @@ def _check_results(grouped):
                                           lambda x: x.weekday()])
         _check_results(by_mwkday)
 
+    # issue 10355
+    def test_std(self):
+        df = pd.DataFrame({
+                'a' : [1,1,1,2,2,2,3,3,3],
+                'b' : [1,2,3,4,5,6,7,8,9],
+        })
+        result = df.groupby('a',as_index=False).std()
+        expected = pd.DataFrame({
+                'a' : [1, 2, 3], 
+                'b' : [1, 1, 1]
+        })
+        assert_frame_equal(result, expected)
+
+
     def test_aggregate_item_by_item(self):
 
         df = self.df.copy()