From dfd44651cc2a08c3047c83c1c35fca3ea9a4f17e Mon Sep 17 00:00:00 2001
From: terrytangyuan <terrytangyuan@gmail.com>
Date: Sat, 12 Sep 2015 20:27:34 -0400
Subject: [PATCH] BUG: Fixed bug in groupby.std changing target column when
 as_index=False (issue 10355) (+1 squashed commit) Squashed commits: [f222db1]
 BUG: Fixed bug in groupby.std changing target column when as_index=False
 (issue 10355)

---
 doc/source/whatsnew/v0.17.0.txt |  1 +
 pandas/core/groupby.py          |  5 +++--
 pandas/tests/test_groupby.py    | 14 ++++++++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/doc/source/whatsnew/v0.17.0.txt b/doc/source/whatsnew/v0.17.0.txt
index 3b3bf8cffe41b..d03bbf7291ab8 100644
--- a/doc/source/whatsnew/v0.17.0.txt
+++ b/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`)
diff --git a/pandas/core/groupby.py b/pandas/core/groupby.py
index f34fd6e3d2575..c6717c16f3e0e 100644
--- a/pandas/core/groupby.py
+++ b/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):
         """
diff --git a/pandas/tests/test_groupby.py b/pandas/tests/test_groupby.py
index 97b57690ccc49..83178970767a4 100644
--- a/pandas/tests/test_groupby.py
+++ b/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()