BUG: STD modifies groupby target column when as_index=False

[jxrossel]

xref #14547 for other tests

In pandas 0.16.2 (and already in 0.16.0), using std() for aggregation after a groupby( 'my_column', as_index=False) modifies 'my_column' by taking its sqrt(). Example:

df = pandas.DataFrame({
               'a' : [1,1,1,2,2,2,3,3,3],
               'b' : [1,2,3,4,5,6,7,8,9],
})
df.groupby('a',as_index=False).std()
Out[5]: 
          a  b
0  1.000000  1
1  1.414214  1
2  1.732051  1

The square root values of 'a' are returned instead of 1, 2, 3.

INSTALLED VERSIONS

commit: None
python: 2.7.9.final.0
python-bits: 32
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 45 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: fr_CH

pandas: 0.16.2
nose: 1.3.4
Cython: 0.22
numpy: 1.9.2
scipy: 0.15.1
statsmodels: None
IPython: 3.0.0
sphinx: 1.3.1
patsy: 0.3.0
dateutil: 2.4.0
pytz: 2015.2
bottleneck: None
tables: 3.1.1
numexpr: 2.4
matplotlib: 1.4.3
openpyxl: None
xlrd: 0.9.3
xlwt: None
xlsxwriter: 0.7.1
lxml: None
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 0.9.9
pymysql: None
psycopg2: None

[jreback]

Something like this would fix it. care to do a pull-requests (and add some tests)?
(should remove the other function definition as well)

diff --git a/pandas/core/groupby.py b/pandas/core/groupby.py
index 4abdd11..3fd2436 100644
--- a/pandas/core/groupby.py
+++ b/pandas/core/groupby.py
@@ -1812,6 +1812,10 @@ class BinGrouper(BaseGrouper):
         'min': 'group_min_bin',
         'max': 'group_max_bin',
         'var': 'group_var_bin',
+        'std': {
+            'name' : 'group_var_bin',
+            'f' : lambda func, a: np.sqrt(func(a)),
+            },
         'ohlc': 'group_ohlc',
         'first': {
             'name': 'group_nth_bin',

[jxrossel]

Hi,
I'm kind of a newbie here (and in Python in general). What do you mean by pull-request ?

[jreback]

see contributing docs here

[jxrossel]

woaw, I didn't consider becoming a code contributer when mentioning the bug. I don't think I would be the correct person for that. I'd probably do more damage than good.

[jreback]

best way to start! give it a shot.